So these people are your DC not your DP, but aside from that they're basically cocklodgers!
There's: You, DH, DS18 + GF + baby, DC27 and DC28? 6 adults? Split the bills 6 ways then and all pays a proper share. It'll be cheaper for them than moving out. As you can't afford the mortgage that needs to be included into the bills before they're split, as it's a cost you wouldn't have if you downsized. If they don't pay up I'd change the locks and say only the baby can stay as long as you can claim the CB for it.
Don't let them take the piss. Stop doing all their chores. Make a chores rota. They should help clean the house not just do their own laundry and washing up.
Speak to GF and make it clear she's a guest in your house, not a tenant, and doesn't get to say when you can/can't carry out repairs and decorating to your own home. Change the way you say things, tell them, don't ask permission. Don't ask "is it ok if I'm in the room to fix/decorate xyz this week?". Say "you'll need to go to another room in half hour because I'll be in this one for 3hrs fixing/decorating xyz".You own the house, all these people need to fit around you.
At the moment they're all taking you and the home for granted, it needs to stop. If they refuse your new rules or ignore them, give them one month notice to leave in writing, which they will hopefully see is serious and change their ways. If not, when the month is up change the locks. That will get through to them when they're sleeping on a friend's sofa!
Of course if you want them gone and to downsize, just give notice to leave in writing and then when it's up change the locks. They don't have squatters rights or anything, you can legally do that. It's highly unlikely they'll be on the streets, they'll have friends they can stay with short term until they've sorted themselves out. Plus they'd have had the notice period you gave to get started on finding somewhere/claiming benefits/getting a job and whatever else they need to do.
You could offer to store their possessions for a while longer if you want to be helpful but I'd put a time limit on that too eg 6 months max. Otherwise you'll still have their things in your home years later.
You could offer to have the baby only live with you to make things easier for them short term. Long term it won't help them if they go after council housing, the baby being homeless too would give them higher priority. If you keep the baby, claim the CB too, it's only fair and you've funded them all long enough.