Check out some tips: There will be times you might not even be second (or third! Just know that her child's schedule, meals and needs will come before yours.
It's natural to be selfish and to want her attention on demand, but would you want it any other way?
This will serve to reinforce your position with facts, and let both of you know when it is time to meet the child.
Incorporate the child into activities as much as possible after the relationship solidifies.
The unsteadiness of being involved in a relationship with a single mother can be trying and cause undue stress in a relationship that would otherwise thrive.