Hi, have been out buying a bike for dd this morning.
It seems a lot more complicated than it first appears! Going by Halfords age guide, a 5-7 would need a 16" bike. (dd will be 6 by christmas)and we were all for doing that, but went to a great independent shop this morning, and even though we had dd inside leg measurement, he pretty much wouldn't sell us a bike until dd had been into the shop and tried some out.
He said that when we came back, we should pretend we were buying a bike for ourselves and they would innocently ask dd if she wanted to try some out whilst she was waiting. He says they do this all the time. It depends on how old your kids are, as older ones will def be suspicious!
The thing is you can get different frame sizes, different types of frames and also each childs reach is different too,so the height of the wheels might be right but the handle bars might be too far away etc. At this shop they customise it, ie adjust the heights, move the seat forward or back etc at no extra charge.
Having said all that, you could get them to try out their friends bikes, and just see if they look about the right size.
Helemet wise, they are measured in cm. DD is 50cm, and it might be worth measuring as they do helemets which are eg.46-52cm, then 50-56cm, so you'd want the one they would be able to wear longest, so although the 46-52cm would fit dd, we'd get much more use out of the next size up,IYSWIM.
Knew nothing about bikes a week ago, so very possible some of the above is not entirely accurate.
Hope this helps.