Just curious – when Welsh-speaking infants are learning to talk, are there sounds that they tend to have particular trouble with, and would these happen to coincide with the ones that come least naturally to English learners, namely ll, ch and to a lesser extent rh? Of course, English infants have their difficulties too. I remember our eldest child was quite late mastering th and r. Being kindly modern parents who believed only in praise and encouragement, we would never have dreamt of correcting his ‘fings’ and ‘wabbits’, so it was a bit ironic that when his younger brother got to the same stage, the four year old subjected the two year old to stern reproof and ridicule at any hint of a mispronunciation. Result: the younger sibling acquired a flawless utterance pdq. There’s a moral there somewhere, but don’t tell Aran…
I studied this a little bit (very much tip-of-the-iceberg stuff) whilst at Uni - it wasn’t part of my main courses and it was 15 years ago now, but I seem to remember something about the order babies learn to make sounds as being pretty general across all languages and to do with which parts of the mouth are used first. If I remember correctly, labial sounds where the lips are the main things moving come out first, and sounds that rely more on accurate placement of the tongue come later on. It’s one of the reasons babies almost always say ‘mama’ before ‘dada’.
I had an excellent book by the marvellous David Crystal on it, but I think I passed it on to Aran.
I was interested in linguistics and the development of languages at one stage in the distant past, but I only remember the labial first point which @siaronjames made and, I think that vowel sounds are easy to pick up, but if your language/dialect doesn’t use a sound, well you don’t learn it and trying to pick it up later on it much harder, unless, I guess you are a natural mimic!
Not that I’ve noticed - ours both took a bit of time to get S working, but that’s the only one I can remember. Maybe R a bit, too. I think the conceptual experiment that what is difficult in Welsh to English speakers also happens to be naturally difficult to everyone is probably something of a dead end… 
Just realised I didn’t actually answer the question!
Infants learning Welsh sounds are likely to have trouble with th and r just as much as ll and rh, because they require accurate tongue placement, but as Henddraig says, sounds that aren’t included in your native language(s) are always going to seem difficult because you generally only come across them much later on, after the prime stage for learning sounds i.e. infancy/childhood.
I think research suggests that most adults learning new language sounds struggle because the neurons that did that so naturally during childhood have become dormant, so it’s more of an effort to re-boot them.
That has set me thinking, Siaron! I was 3 to 4, I think, when we went to Northamptonshire to be near my dad at the POW camp. The Northants accent can be pretty deep! My snobby Mam had been raising me to sound like the Royal family or a very posh actress, I learned a lot of German and loads of new English accents all at once, because the Army folk at the camp were all mixed up, different regiments, different War service. Anyone just repatriated and not yet discharged might be sent to any of the different camps in UK, Germany, Israel, Cyprus etc. I bet I got my ‘sound copying’ neurons turned on full strength and it was not so very much after that we moved to York and I learned to fit in there… I winder if mimics in general have that sort of youth?
My old Catalan tutor reckoned the really trilled -rr- was the last sound native Catalan (and Spanish) children mastered, and the first to go when sufficiently drunk – so No estic bodatxo would have the same sort of effect as “I’m not pished”.
Ooh, I’m going to have to practise that…
If I remember linguistic acquisition principles correctly babies make random sounds while developing muscle control. It is the people around responding to the sounds as if they are intentional and repeating back those in the local language that encourages the use of particular sounds. That’s what all the repetitive baby talk is about. Babies like the power they gain by eliciting responses in others and so drop the unused sounds.
my now nearly 9 month old grandson is becoming an expert in “mama”, and quite good at “dada”.
I think he does a passable imitation of “hello”.
It’s a fascinating process. Something of a minor miracle, I’ve always thought.
I have mentioned elsewhere that I remember learning to say ‘Dadadadad…’, OK, I am 100% sure my Mam encouraged me! She was desperate to be sure I’d know what a dad was, that I had one, that he loved me, that he had not chosen to go to this place called Burma with monkeys and men with guns… But it is my memory of lying in my pram watching the canopy dance slightly in a summer breeze and trying to get the right noise and the feeling that I had when I did it!! I did not know the word ‘triumph’ although i know I knew words I could not say, but I can still taste that feeling and know I yelled ‘DADADADADADA…’ at the top of my voice! I must have been 7-8 months old or so.
My dad has always reckoned that my son has quite a sense of humour. When they were visiting us for his second birthday my mum was trying to get him to say “Grandpa Jim” – partly, I think, because she was a little miffed that he called all his grandparents “Mack” (a close approximation to his other grandfather’s name, Mike) without exception or regard to gender.
The exchange went something like this:
Me: Can you say “stegosaurus”?
B, quite clearly and comprehensibly: Stegosaurus.
Me: Can you say “brachiosaurus”?
B: Brachiosaurus.
Me: Can you say “Grandpa Jim”?
B: No!
Sometimes I think it’s not so much about articulation as the desire to learn
Wow, that is an amazing memory. My husband can’t remember anything before six, but as I don’t think it was a happy time maybe just as well. My first memory is being excited at a photo being taken of me. My Dad tells me he said “Stand still over there and I’ll take a photo.” In a typical response the phot shows me running as fast as I could towards him! I can remember the excitement as the camera was borrowed from my Uncle. I think like you it’s the excitement that keeps the memory fresh. I was two.