Recently my girl age 5 has been complaining that her panty keep sticking on her backside or go in between the backside. I already buy her a bigger panty but don't seem to help. Anyone has idea to avoid this? Thanks.

Instead of bigger size panties, Get those full covered panties that cover the butts. it will be better then those narrow shaped kind.

Don't get her panties, sometimes too big also.irritating. why not use boxers or teach her to adjust.

Probably the position she is sitting. Or You can try getting those boxer shorts type

I prefer boxers to panties especially for younger girls.