How do you navigate babysitting overnight?

I babysit for one family (not a family friend or something like that, very much a client) and they had me babysit overnight.

My rate is 20$/h and they had me over for 26h bit I made them a price, 400$ (Canadian currency) for the weekend. (There's 3 kids, 5, 8 and autistic 11, I'm 22, they specifically wanted to hire an adult with experience with autism)

Now they want me to babysit another weekend overnight and the told be "usually babysitter do a fixed price for weekend because at night since everyone is sleeping it's less $ :)"

And I might just end up saying I have plans those days because I work full time, I don't want spending my weekends babysitting becoming a regular thing.

How would you navigate that? How much would you charge for a overnight babysitting?

Thanks,

Frequent comment: I really don't think the parents are getting money for the autistic kids related to me taking care of him.

This regular under the table babysitting, in not a childcare worker I usually do a night every other week and some weekends

20$/h is in the higher part of standard pay in my area, childcare cost almost nothing where I am