If you or your Kids ever use a bicycle, then you should always use a cycle helmet. There is no excuse really, I know they do not look "cool", plastic, but neither does a Kiddie with brain damage!
My helmet has an outer shell made of Polystyrene, making it almost weightless, there are Nylon webbing straps to keep it secure. My helmet is actually ex Royal Mail, I found it for sale second hand, and although I would not normally recommend the used of second hand safety equipment, I will make an exception in this case.
The outer shell is wipe clean, the inner can be swilled out with soapy water if need be, and the whole thing can be dried with a cloth ready to wear. There is also a ratchet system fitted inside, meaning you can crank it up until the rim is the perfect size, so no problem with unusual sizes there then.
The helmet will probably last for ever, and the only reason I would change it, is in the even of an accident. These helmets are very comfortable to wear, and the shell is vented, so they are "cool" after all!
Many riders are injured every year, quite a few are even killed, and in the majority of cases, a helmet would have saved them. You and your Kids are well worth the cost of a simple helmet.
I am sorry this is quite a short review, Thank you.