I wasn't exactly sure what to call this but I wanted to share with you something that has helped my family out. Caden no longer wants "baby" cups (sippy cups) but he would sometimes spill regular cups. After thinking over the problem for a few days, this is what I came up with. It makes him happy that he has a regular cup and I'm happy because it doesn't spill as easy. It will still leak if knocked over but it is a slower leak through the straw.
What you need:
Press 'n Seal
Fill the cup with desired drink and cover with the Press 'n Seal.
Take the sharp knife and poke a small hole in the middle of the Press 'n Seal. Ideally you want the hole to be smaller then the straw so you can wiggle the straw in for a tight fit.
Place the straw in the hole.
I hope this tip helps your family keep the peace as much as it has helped mine!