Our younger son is in the 9th grade and has been in a Spanish Immersion program since kindergarten. Technically he has been considered bi-lingual/bi-literate since 5th grade. We took him to Guatemala for a mission trip just after 5th grade. We spent a week living at an orphanage, with lots of interaction with kids his age. He was very shy about speaking Spanish, even though he easily understood what was being said.
Three years later, we went to Spain. At this point, my older son (16 at the time), had been taking Spanish for 3 years. My older son REALLY enjoyed listening to people speaking Spanish. He was surprised at how much he understood. My younger son was still shy about speaking Spanish, but he did very well as our interpreter.
I think it’s a great idea to travel somewhere that speaks French, just don’t expect the kids to engage in French. It’s more about cultural exposure, as it would be with any non-French speaking tourist. The kids may also feel very overwhelmed, and possibly disappointed, especially if they aren’t very familiar with “native” French. They might pick up a word here and there, but don’t expect too much.
As for best ages for travel, my kids are 3 years apart and I think the best time is about ages 11 and 14. They will have more individual interests that you can focus on, they’ll be more independent, and they’re still considered children so you can save some money on tickets.