Oh, my first diaper had the same booty!!! Also from a pattern I drafted. I think it's because I used those pattern drafting instructions, but with measurements that had a wider waist and shorter rise, so just making a random, symmetric curve was not a good idea in this case! I had to really make the booty alot wider. It took me about 3 revisions to get it to where I really wanted it, but only the first one just isn't useable.

When you measure rise on your child, make sure you measure with NO diaper on - that might have been your problem there?
For elastic, there are two problems you can run into. One is not stretching it enough. If you're using polybraid, stretch it as FAR as you can. You won't break it.

The only elastic you can't do that with is Lastin (the clear stuff). Also, if you're doing a casing, make sure you do NOT catch the elastic when topstitching through that area. That will cause it to not retract in the places you caught.
I recommend taking some scraps of the materials you plan to use, and practice putting in elastic. It can be short pieces (like 3 inches unstretched). I have a few of these running around... fleece and flannel with elastic, PUL and cotton print with elastic, etc, etc. By using scraps, I was able to figure out elastic. One time it just suddenly clicked and I did it right! Although, when I do a casing, I do NOT put the elastic in first and then sew the casing around the elastic. I sew the casing first, leaving the ends open, then thread my elastic through with a safety pin and tack it down on both ends. MUCH easier to get it right every time.
