Sounds like there was some undue pressure put on the seams. Were the pajamas a little small on you to pull at the seams like that?

I generally do french seams when working with delicate fabrics to avoid fraying. I've never had the experience of the surrounding fabric splitting as you've described.

If they're not too small, is there enough fabric left to do another row of smaller stitching on the non-frayed part of the fabric body? Maybe someone else will come up with another solution.

It stinks that you put all that work into the project and there's a problem.

If the fabric was not cut on the bias when she cut the pjs out, it could be the problem.