I tend to agree with bdrm. And I'll add a couple more points to consider.
Dating single mothers will always be problematic. Her kids will ALWAYS take priority. And I don't think you'd be vibing with a woman who did not make her young child her #1 priority. Now kids, especially younger ones, can be unpredictable. Unless you think she was using the brat as an excuse and there was no problem at home, you might want to grant a little grace here.
Now it's possible she was actually playing you and she really doesn't want to do double-plays for some reason. Maybe she's not really a two-pop girl and just wasn't into another round that day. Like bdrm, I am also a one-and-done guy, so why can't a SB be one as well? Or maybe, she's internally closer to the 'ho side than she lets on and she thinks her current allowance is only enough for one ride per date. She may be leery of telling you that, fearing you will break it off.
But as my post title suggests, one event is not a trend. I think you might want to reiterate your desire for a double-dip and see what happens. If she agrees at first, then pulls the "gotta go" bit, you will know what to do. If instead, she tells you she will be more comfortable with an extra $ or $$ allowance, you can make a value-based decision. But if she agrees, shows up, and gets down and down again, then...
Life is good
The Cat