A good way to keep a diaper on a toddler when he or she takes it off is to use some duct tape or packing tape around the waistband of the diaper.
Just don't get the tape on the toddlers skin.
But after putting the diaper on your toddler take a roll of duct tape or packing tape and wrap it around the diaper at least once or twice if your toddler is one to easily pull tape off.
The duct tape and packing tape are harder for the toddler to remove.
Or you could use a cloth diaper cover with some snaps that are harder for some toddlers to remove on their own.
Another option is to get some Pajamas that zip up the back and then use a safety pin to help keep the zipper closed.
Or by some harder to escape pajamas.
I've used duct tape on my toddlers diaper and also had to put some duct tape around his pajamas so that he could not get to his diaper.