Hi rio :o)
Hmmm if it is just their signiture that is required and not that of an adult, like my 2 children have (both Hubby and I must sign to be able to withdraw any money and it will remain this way until they are 18) then I would get in touch with the bank and explain what is happening. Surely this is similar to losing the books? Just ring or go into your local branch, get the old cancelled and new ones sorted.
Hope you manage to get it sorted or that someone else here who is more knowledgable than me can help you :o)