The younger one is only a toddler - but the older one certainly mixes words from each language (Japanese/English). Sometimes he doesn't know at first which lang

The younger one is only a toddler - but the older one certainly mixes words from each language (Japanese/English). Sometimes he doesn't know at first which language a word is from, and only learns later. I don't really think any of that really matters very much though, honestly. People think of languages as distinct, separate things, but in practice I find that our brains treat them more as collections of words and ideas. If you listen to kids who are bilingual in the same pair of languages, you often see that they freely mix words from both - which is fine since they both understand both. I do this sometimes as well. The most important thing is learning those collections of words an ideas through exposure. For kids, I find the ideal is to speak with other kids. Speaking with parents/adults is good too, but (as I said) it should be natural and not forced.