8 years old is old enough to know better OP. I feel that 8yo is misbehaving for attention.
Firstly, you need to have proper childcare in place. Even for a few hours per day if you cannot afford the full day. Leaving kids with a person with a disability who cant leave the house is a recipe for disaster and they are likely left to their own devices (no shame on the babysitter, theres nothing more they can really do!)
Failing that, if childcare really cannot be afforded, you need to plan activities better with babysitter and provide the activities/supplies when they are dropped off along with a schedule. Make sure babysitter adheres to the schedule! This can be arts and crafts, puzzles, screen time etc. Lay it all out. Kids do well on set schedules.
At home, its important to set boundaries for bad behaviour and also reward for good behaviour. Positive reinforcement.
For now, at home, DD who is misbehaving is not allowed her friends in the house and can only go outside if she behaves. If not, she can't see any friends. And she has to come inside and help with chores. If she doesnt comply, you send her to a room with no devices etc until she is ready to apologise and continue the chores. Once chores are done, you tell her well done, and allow her outside again. Rinse and repeat.
These chores can be anything - hoovering/brushing, drying or washing dishes, tidying her room, dusting around the home etc. Also get kids involved in cooking etc. It keeps them busy and they learn life skills.
The issue nowadays is that parents are emphasising on the fact that "kids should be kids" too much. Not every day needs to be a fun day, especially if both parents work.
When I was little, I didnt go to summer schemes very often. I had opportunities to go outside and play, or I had books, TV etc inside if I didnt fancy going outside. But I also had chores from a young age also. I had to make my bed daily, tidy my room, help with cleaning around the house and laundry. I was tasked with putting dinner into the oven from about 9 years old!
My 8 yo can be challenging also, especially as he is ND. He thrives off having something to do. If he says he's bored, I give him a cloth and let him dust surfaces in the house, or I get him to brush or hoover. He helped wash the car the other day. He also has to help and observe dinner being made every single day and help set the table. If he does well, he gets a treat or a fun day doing activity he wants at the weekend.