Language issues for kids
Been through it myself, put my own kids through it. Yes, do it. With no hesitation. It's one of the best things you can ever do for your kids.
Some books on the subject of bilingualism (which is remarkably understudied, despite affecting 1/2 the world's population) are available at your local library.
> What can we do to make adaptation smoother?
Home language at home, school language at school. Be strict.
If you have the choice (Brussels), then French has the more rigourous grammer, but Dutch has a wider soundbase, both of which are important in further language learning.
If you home language is English, then your kids will be fine in either language. If it's Japanese, then you're going to need to supplement them with regular Japanese language lessons from the Japanese school.