It's actually much easier for children to learn multiple languages while they are very, very young. Children in pre-school and kindergarten who speak only Spanish learn to speak English very quickly...they are often fluent by the end of the school year. Children in higher grades (but still early elementary) have a much harder time learning English. It is not unusual to see a family who only speaks Spanish at home that has a 6 year-old that is bilingual, a 10 year-old that is struggling, and a 12 year-old who can't speak English at all.

I say speak both languages to them as often as you can. Maybe even say something in English, then immediately repeat it in French...ask them to try to respond in the same way. You could even make it a game.