YANBU at all - as your name suggests, you're not a childminder right?
Would she be expecting you to child mind her DD through the hols without paying you? Do you get paid for collecting her DD 3 days a week??
Definitely NBU IMHO. I think she's being quite cheeky to even suggest it, as if you'd wanted to, you'd have offered. In an emergency situation then it's different, but just because she lives next door, that's not enough - and if she doesn't get on great with your child, then again definitely not.
The biggest thing to consider here as well is that this lady is your neighbour, so if you were to fall out over child care arrangements or something that happened between your DDs - you can't get away from it.
Just as an example, when my DD1 started school, I made friends with another mum on the playground - thought she was lovely and we regularly shared a coffee etc after the school run. Our DDs played together, but after a while, I noticed my DD saying things which didn't make me altogether happy about their friendship; ie other DD was mean to my DD and oftened pinched her or pushed her etc. Anyhow, we encouraged our DD to make sure she'd got other friends to play with too, carried on talking to other mum and sharing a coffee etc, but then it appeared that her DD had been saying things about my DD etc. I was very open and said " you know what, they fall out a million times a day, but it doesn't mean we have to!! If we fall out over them falling out, we'll be the ones that look daft...."
She agreed and that was that, we carried on having coffees, and then one day I turned up at school and she blanked me completely. Then she started telling other parents how awful my DD was etc. Very very sad, but from then to now we have never spoken again. She moved out of our village a few weeks ago with DD and DD left the school. Very sad, but she obviously wasn't as nice as I thought she was. Still a shame to fall out with her though.
My BFF has a DD a year older than mine - at the same school etc. They get on great but also have their spats. Me and BFF just accept that they do not get on all the time, and there is an age difference, and that's fine. Both girls feel comfy enough in each others houses to just veg on the sofa if that's what they want, they know they don't have to be in each other's pockets just cos mummies are yaddereing to each other again!!!
YANBU - your house, you choose who comes to play!!!