I’m trilingual, basically grew up on one majority language at home interspersed with another lesser used one, and learnt to read and write that lesser used one and another one at school. I’m very fluent in all three, and can read and write very well in two of them.

You would have to be careful your child learns the language well. Most of my friends growing up would be in the same situation as me, but unfortunately because they were more likely to have a friend circle who speaks majority same language as they do at home (which is not learnt formally at the school we went to), they end up being weaker in all languages as they won’t be comfortable speaking the ones that they learned formally (less practice, only spoken in school), but are fluent in their home language but cannot read and write in that language. I didn’t have friends who spoke my home language (uncommon dialect where I grew up, but very common where I live now), therefore I hung around with the crowd that spoke those two formally learnt languages, and I was also a total bookworm, that’s why I ended up being fluent in all three. I hope that makes sense…..