My DD liked Boowa and Kwala - which is in French and English (you can choose language) and is age appropriate for your DCs. Here's the link boowakwala.uptoten.com/kids/boowakwala-home.html
Another idea would be to pop into your local supermarket and pick up one or two of the many educational/"fun" workbooks that are very popular with French parents. They are all available dependent on the school years eg; CM1 etc.. So you can go through these.
Another thing that helped my DD was watching cartoons and/or Disney films in French (especially ones she already knew in English) - so Franklin, Tintin or whatever (sorry DD now 17 so not sure what cartoons are popular now :) It helps that French DVDs have English language option (so buy in France if you can - try a vide grenier for cheaper ones)
Don't worry about getting their French up to speed too much - make it part of everyday life if you can - watching telly etc... Personally I don't think it helps too much as you can't really prepare for the reality that is being immersed in a a new language and culture. I speak as someone who spent hundreds of pounds preparing DD for our move (over two years) - she recently said the lessons were next to useless, although she did like them and her teacher! The best thing she says was learning French words for things she would be told to get out at school e.G: pencil sharpener, rubber, pen, paintbrush, text book etc...
I would also be wary (unless you or your partner are French) of using French in the home too much - home can be a sanctuary from the pressures at school and if you are not fluent then you will give them bad habits and worse be ridiculed when their French is better than yours ;)
They will pick up the language in the playground and classroom and there will be enough "learning" at school to start with. Your best asset will be to try and ensure your French is the best it can be to help them when it comes to homework and speaking with class teachers - apologies if you are already fluent.