It doesn't matter which background music your children sing along to in your final performance. (Being a bit of a traditionalist, I don't think you can beat live music of any kind, but backing track CDs/mp3s, I'll admit, can often add a bit of pizazz!) However, please make sure that you don't change from using the CD/mp3s for your rehearsals to using the piano for your final performance, without your children having extra rehearsals with the piano, just to get used to the different sound and style. Without those rehearsals, your show could be disastrous, as the children forget their singing and (especially young children) try to loudly inform you, during the performance, that the music is different to before and that they don't like it! (It happened to me - once only!)
Also, if you are using CD/mp3 backing tracks, make sure you have the sound turned up high enough for your audience to hear it and for the music to have an impact, but not so loud as it drowns out the children's singing.
Oh! And ALWAYS have a second CD and CD player (a reasonable quality portable battery operated one, which will play loudly enough if necessary - some CD players are woefully quiet and tinny...) ready to play, in case of unplanned technical hitches e.g. scratches or cable faults. They do happen.