My 0S definite has that same problem, and I came up with a very simple solution, albiet it's a bit crude. If you got any toilet paper in your house, just take a small piece of it and try to separate the layers until you're down to the smallest layer possible. Simply place the small and thin piece of toilet paper over the backpack hole before you push the leg pegs in, and the fit should be more than tight enough for you...errr, i mean your kid...to zoom it around any time you want.
It's ghetto, but to be it's a simple solution that to me is preferable to, say, gluing the leg to the backpack, as i can remove that thiny, thin piece of toilet paper whenever i want.
Just my 2 cents