First of, don’t worry or panic! It’s very normal for a child to prefer to play than sit down and practice!
 
A number of factors may contribute to your child’s reluctance to music, the most basic of which is that for a young child sitting and practicing an instrument is not a natural thing to do! If it’s not a game for them, they will most likely reject it, so it’s natural and don’t worry thinking that it’s only your child who refuses to practice. You need to turn practicing into a game, and make it fun for them. Here are a few tricks:
 
Pretend to attempt playing the instrument, fail "miserably" at it and ask your child to show it to you how it should be done
 
Close your eyes and say "let me see if I can tell if you played the right note or not"
 
Ask your child to close the eyes and try to play and see if it’s possible (then when playing with the eyes open, it becomes an easier task, hence less resistance to playing)
 
Tell them that you don’t think they can play this line five times; often children to prove others wrong, they just do what they were refusing in the first place. Reverse psychology can play a major role in such situations; there are many books on this subject that you can consult.