Blisters can be caused by either burns or friction. Often, kids get blisters with new shoes.
The outer layer of skin separates from the inner layer and fluid from the inner skin seeps out making the area burn. Eventually, the fluid inside the blister is reabsorbed and the dead outer skin peels off revealing a new layer of healed skin.
If your child has got himself blistered, do not prick it. The skin underneath will be raw and sore. Put a ring shaped gauze over it with an adhesive dressing or a corn plaster to protect it from firther friction. Check the shoes that caused the blister. If it does not fit your child properly, discard it. If the child got blistered because he wore it without socks, try them on with socks, but only after his foot is completely healed. Meanwhile, let him go barefoot as much as possible because any rubbing on the blister will be painful.