I used this trick for my 4 year old and recently had to resurrect it for the almost 2 year old. It is always a struggle to keep little ones from opening their diapers, but you want to keep them cool, especially when you live in HOTlanta. I purchased the smallest size boys underwear I could find (2T) and I slip them over his diaper when he's around the house or napping. The underwear fit snug enough over the diaper that he's unable to pull them down to get to the tabs. As an added frugal bonus, you can use the same underwear when the time comes for potty training.
For other great tips, check out Rocks In My Dryer.