If you aren't giving up any important event, I don't see a reason why you shouldn't do it. She asked you four weeks in advice, which is respectable and considerate. She doesn't ask all the time, so she probably used you as a last resort.
Also, if she doesn't bring it back up, I would just leave it be. Since you didn't say yes when she first asked, she probably felt that you couldn't/didn't want to do it, so she probably found someone else to watch him.
I don't see a problem in you helping her out.
M:37 W:37 T:11 M:10 S17, S13, S10, S4 BD:06/28/17 OM confirmed 07/20/17 Recon the M 10/29/17 Working hard:2gether
Onward and forward
This process is not a sprint it's a marathon! Patience, Patience, Patience.