Welcome back @Dysgu-Cymraeg! Abd if it was you who "dragged" Tatjana to us, diolch yn fawr!
The kids will have absolutely no problem picking up Welsh in school.
In my experience, the problem comes later in school life when the kids start to understand that this Welsh stuff only happens in school, and doesn't seem to fit in anywhere in the real world. It's a common problem in areas where Welsh isn't widely spoken, and is even a problem in south west Wales where Welsh is still common, but not essential.
By learning Welsh yourself, you will achieve two things. One, the unborn child is already learning a "mother tongue", so hearing you speaking Welsh (yes, even 'poor' Welsh with lots of "Oh shut up Cat I'm not ready yet"s stuck in) will reinforce Welsh as part of the mother tongue before you ever get close to school.
Secondly, you will create a habitat where Welsh can exist outside of school. If you insist on speaking some Welsh (and especially if you make Welsh a part of your daily life), then your child will understand that there is more to this Welsh thing than just school, giving them a better chance of valuing the language, improving their language skills, and improving their enjoyment of school activities.
Schools in less Welsh-speaking areas are set up to deal with second language speakers whose home language is English, so your child would have no problems even if you never uttered a word of Welsh. But show them that Welsh is valuable by making the effort of learning / using / enjoying Welsh, and you'll set them up for an even better experience.