From a very young age (I don't know, 6 months or younger), Evan learned that if he put his fingers near the tape on his diaper and wiggled them around, he could get his diaper undone. A few months ago, he perfected the technique.
It doesn't matter if he has a onesie on; he just reaches his hands through the bottom of them, peels back the tape, and slides the diaper though the leg opening. It happens during the day, and at bedtime. It's a guarantee that I'll have to put the diaper back on him at least once after I put him to bed. He's usually awake for at least 45 minutes after he goes to bed---just playing....and taking off his diaper. So, while he is still awake, I go and put it back on. Chances are, it will be back off soon after I leave the room. Sometimes, it's not a problem--he stays dry. But of course other times, we have to do a complete change of sheets and blankets.
He has one pair of overalls that he wears during the day that he can't get his hands down to maneuver the diaper (he wears them frequently). I'm going to have to look harder for pajamas that are difficult to get into. I don't like the one piece ones with the feet on the bottom. I used them for my older three, but chose not to for Evan because several years ago, John bought some for me. My feet felt so trapped in them, and most of those are polyester, and personally, I don't like to sleep in sweaty polyester. So, I've decided, neither does Evan! Many stores have the two piece summer pj's out now; those would be like an open invitation for him to take off his diaper. We need short sleeve, long pants, one piece, that don't cover the feet. Not an easy find. I've considered duct tape for the diaper, but I'm afraid his fingers would get stuck in the icky, sticky tape.
He has gone on the potty twice, but I just don't think he's ready to be potty trained. He'll be 2 on the 23rd of this month. I'll continue to put him on the potty occasionally, but it's definitely not something I'm going to push.