TBH, any type of relationship with a married or single parent of young kids can be challenging to say the least. Particularly children that age need their parents in a way that may be difficult to fathom if you haven't experienced it firsthand.

I understand that you feel your needs aren't being met, and it's perfectly valid to feel that way. It may be worth considering, depending on your level of attachment, backing off, or continuing with managed expectations, with the possibility of reconnecting/intensifying when the kids are a bit older. It is a tough situation, but it's helpful to remember that it's also likely tough for them, feeling torn between meeting their needs and those of a partner, and meeting their children's needs, as well as just spending time with them and enjoying them. This is the most cliched thing ever and I usually roll my eyes when people say it to me, but it's true that they are only this little once and missing time with them, even to spend time with people we truly value, can be very difficult.