Urine: Nuby

Problem with Sippy Cups

I am raising my 2 1/2 year old grandsons, my problem is they drink out of Nuby soft spout sippy cups, which are great. The issue I am having is they do not have the valve in them so they are not exactly spillproof when the children are biting holes in them. Then they will lay them down wherever they are and of course the cups drain all over. I have tried to get both of them to use other cups but they refuse to and throw temper tamtrums until I give them back the cups. I need some advice on what to do at this point in time as they spill...