...assuming you know a second language, which odds are children are gonna learn in school.
Schools pretty much force you to at least know the basics to get a passing grade. The web only helps if you have self-initiative.
The internet makes massive difference in availability of movies with subtitles you can watch over and over, songs, later easy to read blogs. Discussion forums also provide super simple english to begin to read. Books and articles are too hard in comparison, both in vocabulary and sentence structure.
Duolingo is massive thing.
Yes, teacher is necessary too. But if you actually want your kids to learn other language, the web and games available easily for free now make massive massive difference.
For general knowledge, Wikipedia is only useful in handful of mostly European languages.