My wife is trilingual with Thai English and Chinese. I am sadly shit at languages. I came up with a plan that worked involving only one language for almost 5 years. It worked the kids are bilingual and approaching decent Mandarin as well. If you would like more information please feel free to pm as I have posted my approach a few times before and dont want to go on.
As every poster who has met my kids can tell you the plan worked. Is it for all? No, not at all, it took a lot of work and time. Feeling were hurts et al. . But the result is everybody on both sides of the family is ecstatic now.
My inability can be chalked up to laziness in reality. As my dad was bilingual and could read and write Arabic as well as speaking it fluently.