How are you attaching the elastic thread? Is it going in the bobbin? If so, I have no advice because I've never done that. If you are using a zigzag stitch across the elastic thread (the way I would do it) then you pull the elastic thread after you have sewn across it. The zigzag stitch forms a mini casing for the elastic. You need to tie the ends of the elastic thread very well with a lot of knots, or it will pop loose. I would use a little bigger round elastic, like the stuff you use to fasten hats on your head (making an elastic chin strap for a hat) and then tie the ends of it very well. I make costumes for plays, and that is how I would do it for a costume. I would think your pattern would tell you what to do.

If you pull the regular thread to gather it, then it will have no give and she won't be able to stretch the skirt to put it on, the first time she tries to do that, she will break the threads, and that will not be good.

Does this help at all?