I actually have the same problem with Eleanor's dungees, but I've never got round to do anything about it.
Anyway, I have done somethign about it!
I decided that it must be the rise that's the problem, because standing up, the length of the body is fine and the legs are fine.
it's just bending over or sitting down - they pop open with the strain.
I don't know why toddler dungees aren't designed with a bit more fabric in the crotch...
Here's my solution.
The dungarees with their crotch open. I measured the distance between the 3rd press-stud from the middle on each side, and made a triangle shape with paper.
I checked that against the trousers and marked (on the paper) with felt-tip where each press-stud was.
Then I cut out the fabric (an old pair of jeans with a slight stretch in the demin), using the length of the triangle as the straight grain.
Please note that I have not added a seam allowance to the pattern, you don't really need a big seam allowance on the top and bottom because it's going to be overlocked.
The extra that I have added to the pattern is the FOLD-OVER part (see next stage!)
The triangle once it has been overlocked down all edges (do top and bottom and then each side separately)
The next bit is important, because you have to press the edges so that they make a triangle. This is where it's better to have a slight stretch, because in the middle where the triangle converges, you will get a bump. It doesn't matter which way you iron the bump down, as long as it's not too thick.
You could lockstitch around the edge now if you want to. Measure the triangle again against the press-studs on the trousers.
It's important to have the right press-studs for the trousers. these are a 9.6mm diameter press-stud.
I have added the press-studs now - lines up perfectly with the press-studs on the trousers. Please note that the back of the press-studs are on the WRONG side of the triangle, and the working parts are on the right side of the triangle.
that's so that when it's put onto the dungees, you can't see the triangle (honeest!)Then all you need to do is join them together!
See the outside of the dungees (the decorative side) shows, as it would have without the triangle, but the other side (the underneath) doesn't show at all.