I agree with Erno86. something like a Crickett
22LR. They can be had for around $100 and are an excellent choice for a youth trainer. Very safe and the single shot first has the gun no longer able to fire after the excitement of a good hit when kids momentarily can forget their fundamentals (even a mature 5 year old is still a 5 Year old) and second encourages them to focus on good marksmanship and not just blasting away ammo.