Where in the OP did she say it was a long weekend?
Try reading TFT, Tequila. It could have been a long weekend if OP's partner had put in minimal effort into finding more suitable travel arrangements. It's not a question of being ungrateful to her mother, rather feeling that the money her mother gave is being wasted.
Surely even if you'd be leaving early Monday morning, that leaves an evening in Paris for a nice romantic meal out, unless I'm missing something
On a Sunday night, with an early flight on Monday? Really?
You've got childcare sorted for Friday night, so maybe out for a meal/drinks in the pub or whatever?
Wow, what an exciting way to use OP's mother's thoughtful offer which she made so that her daughter could go to Paris, not go for drinks in the local pub.
Childfree lie in Saturday morning, arrive in Paris around teatime so you can have meal/wander along the Seine/see the Eiffel Tower/insert choice of activity here.
Or, of course, for the same amount of money OP could have woken up in Paris and had the whole day there. What's grabby about preferring that, Tequila?
Back to hotel, wake up in Paris, have a full day together seeing the sites you didn't see yesterday, meal/drinks then back to hotel in the evening
You do know that sightseeing usually involves seeing more than the outside of buildings, Tequila? How much do you imagine OP is going to see of the inside of Notre Dame, the Eiffel Tower and Versailles do you imagine OP is going to see on a Saturday evening? Bear in mind also that lots of Paris will be closed on a Sunday.
Not leaving until Monday, day off work all taken care of
Monday morning. So OP sees nothing of Paris that morning, gets back home in the morning, and she's wasted a day's leave and a day's childcare. When she could have had at least half a day's extra sightseeing/shopping.
Maybe it's just me, but I'm really struggling to see why anyone's first reaction to that would be to be angry and cry it's thoughtless, not long enough
Can you really not see that, when OP's mother gave the money and offered babysitting enough for a long weekend, it's not grabby to wish her partner had used the money more sensibly?