If my sister babysits i we give her cash for takeaway and movies etc (and loads of thanks)

Mum looks after MJ on thursdays and Fridays, she does not want/expect to be paid for it.
We give her $50 a week in fuel (she would probably use maybe $25 if that for the travel she doesn) and i will occasionally give her an envelope with $100 in it, its probably about once a month. she is always cross, but always accepts it (which is nice)

My friends are going to have MJ for a day while i am at work and i plan to give them some flowers and choccies and something for the kids as a thank you.

Its about how much do you want her to come back and do it again, if she feels apprecaited (flowers, choccies, wine etc) she will be more inclined...
And even though your bubs will be in bed, its still the responsibilty that she is undertaking... what happens if he does wake? she has to stay sober, in hearing distance and not distracted kwim?

Enjoy your night out