Proud father of 3 and they all speak two languages fluently. in fact 2 of them speak, read and write 3 (English, French and German).
Both parents are mother tongue in the same language but have a good command of the second language.
We mainly spoke our mother tongue as they grew up and they had pre and primary school for what is now their primary language.
We did, however, make an effort in the second language as well. To the extent that the 3rd child when learing to count 1 to 10 did not realise that the different names for the numbers came from two different languages for quite a while :)
We noted that they can be slower at picking up some vocabulary compared to kids who only have one language, reading as well can be slower - don’t panic they are assimilating twice as much and they will more than catch up in time.
The 3rd child expressed concern because she could read in the language taught at school but not in the other... So we got a book in the other language ( a Harry Potter one) and we read that together - I read one sentence, she the next. Ok, pronunciation needed work at the start but the read-to-speech capability was there. She was so pleased :)
2 have sufficient command of the parents mother tongue that they have corrected the teachers (who have then found out that the kids are correct :) ) some teachers take that well, others less so. But if the teacher does not have sufficient command of the language they are meant to be teaching then they need to improve.
Kids learn languages so easily when they are under 10 years old - and I have seen this with my colleagues and their kids as well as with my own. they now converse with each other in what is their primary language and will change in the middle of a phrase without any hesitation if they talk to either parent... and they have been known to correct my pronunciation in my second language - always popular :)