Most of the major helmet manufacturers now produce a range of helmets for children giving as much thought to the colour and design as the fit and safety. Naturally, safety first. When buying a child's helmet ensure it's the right fit and sits as shown in the manufacturers guidelines. A helmet that is loose and moves around too easily will offer little or no protection in a tumble or collision. Check the helmet is EN 1078 European Standard approved.
Very young children that are being towed or transported by the parent need little in the way of ventilation as they generate little heat.
However as your child becomes more active and insists on doing the longer rides with vigour, try and find a well ventilated helmet whilst still ensuring the fit is correct. When fitting the helmet try to avoid over-tight straps or nipping your child's neck with the clasp by sliding a finger behind the chin-strap when fastening. If at all possible try to engage your child in the choosing of the helmet - a helmet they like is a helmet they'll wear!