I think helmet not needed as the kid probably wont strike his head on the floor. most important are the knees and hand gloves as he will fall down easily. my girl is 4 yrs old, she started skating one year ago, she got protection on the elbows and knees and hand gloves. so far so good.
It should be better to wear the head protector because you don't aware some outsiders accidentally slipping to the floor and hit your girls when they are taking lesson. So I will buy the head protector