I have painted loads of murals in the past, both professionally and for my own kids (and grandkids).
Acrylics are definitely the best to use, and as Wendy says, an emulsion ground is best. I always preferred a matt emulsion for the ground, as there's no sheen, but silk is ok.
Acrylics can be easily painted over with ordinary emulsion when you (or the child) gets fed up of the mural. It's best to use relatively thin paint for the mural, so that the wall remains smooth. In this case, cheaper paints are sometimes best.
If you have large areas of one colour, emulsions mix quite well with acrylics. Check the sheen of the acrylic paint if you are mixing with emulsions; some acrylics are matt and some have a bit of a silk sheen, so match your emulsion accordingly. I always used to use white emulsion for mixing.
You can use a waterbased varnish to protect the finished artwork if you like. I wouldn't use solvent based because it yellows and will be difficult to paint over at a later date.
Good luck with the mural, they are fun to paint. Enjoy!