Firstly OP, well done for leaving. You've taken the first step towards changing your children's future. You've also taken the right steps in linking in with support services and therapies!
As others have said, It'll be a long road - if you read about attachment and relational trauma you'll see that your children have likely developed views of the world and the adults around them as being unsafe and unpredictable. Their current behaviours are survival adaptations and learned behaviours.
This sounds scary, but if you keep reading about attachment and relational trauma you'll see that these worldviews can be shifted and changed with careful, close, loving support.
Their priority need right now is safety and security. Without feeling safe, their brains won't be as able to take on the changes that you want to see through therapy and intervention.
To feel safe your kids need a high nurture AND a high structure approach. They need to know you're their safe base and love them unconditionally, while also knowing that YOU are in control and can keep them safe - children experience safety through predictable routines and clear boundaries. They will also push back against these things (as they test the world around them), but it's your job to lovingly and firmly hold boundaries of acceptable behaviours AND to reconnect with them when things go wrong.
They can't be too reliant on you right now - it's really common for kids to "regress" back to younger behaviours like needing you to lie with them to fall asleep etc. That's completely ok. Now is not the time to be pushing independence or being a "big boy/girl" in terms of responsibilities - they need to allow themselves to rely on you. Think of how much emotional support you'd give a toddler during/after a tantrum - cuddles, naming their feelings, explaining that it's ok to be angry but they can't hit etc - that's the type of approach they'll need.
Speak to their school when it reopens. They will likely need a trauma-informed approach from the staff too.
Also, everything I've just said will be intensely hard on you as their support person so make sure to link in with your support network and services too.
Sending love x