I don't know if it's true or how it could have been tested, but I've heard from several couples bringing up bilingual children that it's good if each parent consistently speaks to them in only one of the languages.
I grew up in a multilingual family and it had its negative impact. I don't verbalise my thoughts and it's often very hard for me to express myself spontaneously - I sound more affected than I really am because of that. Another problem is naming things in the language in which I met or experienced them - when I have to translate them to other languages, it ends up with elaborate descriptions of simple things, because I want to express every subtle difference. And finally, I use words which don't exist - ca. 25% of the vocabulary is common (Greek and Latin) to the languages I know, but I'm very creative and make it 50%! It's obviously good that Julian is bilingual, but it's not all positives.