Thanks. This is what I've been doing, but it ends up me repeating myself 30 times while she's like "seriously, anytime. If you want to get your nails done. I'm free Friday night if you want to go out. Or if you ever need to run errands." And on. And on.
I wasn't going to tell her I don't trust her lol, I was thinking more along the lines of telling her I'm not really comfortable leaving Lillian with anyone except her grandparents, but then if she finds out about anyone else babysitting (not that I'd really leave her with anyone else) she might be offended.
Well, it doesn't HAVE to go on and on. She offers to babysit and you say not needed tonight, but thanks! If she keeps texting you again saying "seriously, I'm free!" you don't have to keep responding to her and get stuck in a loop. The nice thing about text messages is they don't demand an immediate answer - or even an answer at all really. It might seem a little rude but conversations do have to end sometime LOL.
Obviously she really wants to babysit for whatever reason. Maybe she really likes babies and wants to babysit. Maybe she thinks a lot of parents don't have anybody to babysit, so she's making sure you know she is available to help, even if it's just for something like taking some time to go get your nails done or whatever (and truthfully I do know a lot of parents who whine about not having a babysitter and having to find somebody.) Or maybe she just really needs some cash LOL. It's quite nice of her to offer, really. And maybe one day you will legit need her in an emergency, so it's nice to know she is willing and probably best not to set anything up to blow up later. The BF might not stick around either.
And yes, if you lie you're setting yourself up to be possibly be caught out later and create an argument and hard feelings. There's really no need to make excuses about it or come up with something that might backfire later. Just a thanks but it's not needed will do the job. Unless she's REALLY bored (or desperate) she will probably stop soon.
Alternatively, maybe she wants to hang out with the baby. Maybe you could tell her you want to run some errands and would she be willing to come along and help out with the baby while you try on clothes/pick up groceries/get your hair cut/whatever? That way she can feel like she's babysitting and helping out and you don't have to, well, actually let her babysit LOL.