Firstly combine forces. Getting her and the children out with all their stuff if possible is a priority.
So on a day he has work -and is out at 8am. You arrive with 10 friends and family and a van or two and cars and as many boxes as you can find and get before off local FB groups etc and empty the house of her property and the children's including beds, clothes, garage stuff etc. no regrets, take all the children's toys, any plants you want etc anything brought "jointly" eg tv, microwave. all warranties etc -all paperwork. You can always give things back but you won't be allowed back to get more.
Then she needs to ensure she has : photos, passports, banks statements and ID, any baby stuff and stuff from the loft.
Regarding online stuff -change passwords on everything but logging in from a totally different laptop. FB, SM, Bank, School, everything, email -change your passwords. Have a standard letter regarding the situation.
Due to domestic violence, myself and A and B have left the address ..............
We have moved to this adddress on a temporary basis..............
Please do not disclose this to any third party.
Then her solicitor -if she has got her ducks in a row -provides a letter saying due to x,y,z, I am leaving you. Please do not contact me except with regard to contact of the children -and that is the only thing and go via my solicitor at this address. Solicitor needs to file an emergency custody order.
Contact the police AFTER this and report the abuse together with any evidence. At this point ask for a meeting with GP and explain the situation and the sexual abuse etc to the GP, police. Notify schools.
Change the address with the post office for her and the children to divert to your parents. Notify school, dentists, doctor etc of change of address to your parents.
It is possible. I did it. I had friends around me that did not waver, once. I regret sometimes, I left christmas dec in the loft for example -I shouldn't of.
Housing, benefits, you can sort out the next day. Once she is out. This is not a consideration right now. She needs to leave asap. If he gets wind of it, he will escalate.