Thank you both!

Yes- stepchildren are eligible for coverage

https://www.opm.gov/healthcare-insurance/healthcare/reference-materials/reference/family-members/

they can too

I'm a non fed currently dating a retired fed. We are talking about marriage. He mentioned that if we were to marry I could go on his health insurance. That's great, but what about my under age 26 kids (2)? He would like me to stop working so we can retire together. I currently carry my kids on my insurance through work. If we marry, are the kids eligible to go on his post retirement health care plan?