Its true!!
I did a dissertation on bilingualism, and all the reading, research and surveys i did proved the point.
Children who are brought up by bilingual/multilingual parents develop a part of their brain (which we monolinguals!! don't). Due to the brain being able to "think" in one language whilst speak in another. This makes cognitive development and language learning in later life easier for bilinguals.
This is not true for children who are taught a second language from a young age in classes. (i'm talking 3 - 6yrs here). Because of the limited amount of time they will hear the second language, and no family to practice the language with, it just doesn't have the same impact, and can possibly cause only confusion.(not harmful, but also not worth while until the child is older and already established in his/her own language).
I did this research about four years ago, and those were my findings. Sadly i have not continued so am unaware of any new developments in the research area.
But all round i think its looking pretty good for your children!! I am very envious, wish i had such a wonderful skill to pass on to my dd.
Fran1