I appreciate my son is an adult but he helps with my daughter a lot with childcare so would that not go in my favour?
No. If DD has special needs and requires a carer and if your DS is that carer, he can possibly claim Carer's Allowance. If you require childcare, depending on your earnings (there is a legal threshold, it has nothing to do with how low you feel your earnings are) you may be able to claim some childcare costs back from the government.
You can simply claim child care fee from your husband as far as what he contributes
No she can't. Her ex isn't responsible for childcare costs incurred during her contact time with their DD. He is only responsible for childcare costs he incurs during his contact time with their DD.
I work a lot of my shifts as night shifts to earn better money so someone needs to stay in the house with her or be able to take her or pick her up from school if I'm not around.
Gosh OP you've got a lot of waking up to do. Many many people are limited in their job choices after divorce. You may be one of them.
If you want to work nightshifts when living in a two bedroom property, your DS could return from wherever he will be living to sleep in your bedroom overnight during these shifts, enabling him to continue providing childcare for you. Otherwise, you'll need to prepare yourself to look for another job working daytimes so you can pay a childcare provider. This is what many single parents have to do. At 13, your DD can make her own way to/from school.
If I was a judge and I heard that you're absent overnight and wanting to shaft your ex for an extra bedroom so you can continue to use a teenage adult son for free childcare, I'd be hearing alarm bells. Not just about your unreasonableness in this whole situation, but I'd be wondering does the 19yr old really stay home the entire time you're out and is he always sober? I'd be wondering is he willing to provide childcare or just being told to stay home for his sister but doesn't actually want to, so is the 13yr old perhaps being left unattended for long periods of time possibly entire nights? Your ex has a two bedroom property and a (daytime?) job, I'd be awarding RP status to him and you could have DD once or twice a week as the non RP. You'd then have to pay him child support payment and he'd claim the child benefit and tax credits (or whatever is available for single parents now).