It is contagious until AFTER the sores have gone away UNLESS they take antibiotics (then after 24-48hrs of the start of the antibiotics). Highly contagious and spread through direct contact with the blisters or anything that touched the blisters (including their fingers that scratched it).

The child that has the blister should remain at home for 2 days after starting the antibiotics. The other child is a bit more tricky...as much as he may get it you can't tell if he will so making him stay home tomorrow may not help anything and will potentially upset the parents. You'd have to go with your gut and hope they are ok with it!!