Pooing her pants because she wants to play

I’m a stepmother to a recently turned 5 year old. She is my partner’s first child and I don’t have any, so we’re learning as we go.We’re having issues with his little girl and her poos. We’ve now had two occurrences where we’ve been at a huge play centre so she’s having tons of fun, and instead of taking herself to the bathroom or letting one of us know she needs to go, she’ll just poo in her undies and continue playing.She’s at the point where she wants to be more independent. She can take herself to the bathroom, she can wipe properly, so we’re just baffled as to why she’s doing this. We think it’s because she doesn’t want to stop playing so she just does what she does and continues until one of us asks her if she needs to use the bathroom.She also hates going to the bathroom because she’s going to miss out. She’ll hold in a wee until she’s almost at accident point because she’s busy playing, and when she’s asked if she needs to go she always says no. I usually just ask her to have a go and see what happens and she’ll end up going, then acts surprised (“Wow, I didn’t know I had to go!”)Her mother doesn’t see an issue with any of this because she likes to say she used to do the exact same thing because she had a fear of missing out, which makes the behaviour okay. We don’t agree with that.Any tips would be greatly appreciated, thank you in advance! via /r/Parenting http://bit.ly/2GLiFem