Break Up!

So... I broke up w my SO of 4 years of y'all have seen the post from before when he was "conversing" with a flat Back bitch. And he told me we needed space and such Bc he was tired of me going thru his phone and accusing him of shit that was clearly going on. 
2 weeks after we broke up I reconnected w an old flame and we are getting pretty serious. We're going on vacation w each other and everything. He wants me to meet his family and such. But he has 2 kids and I'm super nervous. His kids are his world, he has full custody so they live w him. I talk to him and he told me not to worry they'll love me blah blah blah. But more than anything I don't want his baby momma to think that I'm trying to replace her in their life. So what do I do?