It's absolutely normal that kids have accidents again....my son had them after staying dry for 1 month...
All of a sudden he was peeing everywhere....he always came and told me he'd done it.....
I didn't get mad, just made him help me clean it up, had him take his wet pants off and put new ones on.....
It took all together 2-3 weeks with some dry days and some wet days, but we survived....
Now he's 99 % accident free........
It's a great thing that your son wants to keep wearing his big boy underwear....that shows he's willing and you should support that....
I know it's hard, but it will be over some day....
About being too busy playing....that is sooooooo common.....
Just try to remind them to go potty in between.....not like "Do you need to go potty ??"... "NO!"...."Oh, ok.".....but more like "Lets go potty real quick."...."I don't have to!"....."Oh, common, lets try and afterward I will play cars with you, what about that? Let's go!"....
Try to stick with it....just going back to diapers because you are discouraged isn't the best thing to do....
If you keep on trying you teach them that you shouldn't give up just because it's not working out at the moment !!!
Good luck !!!!!