OP I am the same. Always been thin, back to pre-pregnancy weight, by BMI is borderline underweight.

But this isn't a brag because I'm a totally different shape than I was. All the fat from my butt has moved forward into my belly. It sucks.

I haven't put the work into the right exercise like a PP describes though.

I've only had one kid and exercise daily and eat well and the stomach is just never gonna be what it was before kids. And that's ok. Unless you're a swimsuit model and your livelihood depends on this, try to show yourself some grace.