Treat Her Like a Person
Always remember that motherhood is just a part of a person's identity---admittedly, a very large part, but she is also a human being with her own needs, desires and interests. Interact with her and get to know her on the same level as you would other women, and make the time you spend together about the two of you.
Understand that a single mother has a lot on her plate. She more than likely is working and, of course, needs to spend time with her children, maybe picking them up from school or sports practices, taking them on outings and helping with homework. She also needs time for herself to decompress, relax and enjoy the things she likes to do. You might have to work around a very full schedule, so you should avoid pressuring her to spend time with you; just enjoy it when it happens, and you'll likely see more of each other in the future.
Don't Blame the Kids
Children are often the most important part of a parent's life. Single parents in particular are often in a position where the time they do get to spend with their kids is limited and therefore all the more precious. Hence, if the woman you're seeing expresses a desire to spend a little time apart to be with her children, never blame the children. Her kids are precious to her, and children can define the kind of relationships single mothers want to be in--the ones that are best for everyone, not the least of which are her children (imaginingourselves.imow.org).
Ask Yourself About Commitment
As you continue to see her, check yourself periodically for what level of commitment you feel comfortable with. If you blend well with the existing family unit, she and you may decide that spending time together with the children is a positive step. Only offer the level of commitment that works for you, in order to avoid hard feelings on everyone's part later.
If you do feel comfortable, offer support, like spending time with the kids while she has some alone time or runs errands. As with any woman you're seeing, you need to be there for her. However, don't try too hard to insinuate yourself into the family; if it's right, it will happen naturally.