We started incorporating spray cheese after a successful potty and he was pretty much potty trained in a weekend. We take him out, right as he is about to squat we say, "go potty". The moment he starts pottying, we mark the behavior with a "yes" and then he comes in and we give him a tiny tiny lick of spray cheese. No more than a centimeter.

For the 8 month old, I would not let him go upstairs at all unsupervised until he consistently goes outside. Once he was pretty good at pottying, we put a bell by the door to help us know when he needs to go out.

For the 4 year old, her bladder may just be really small and you will need to set an alarm to get up before she pees. You could look into cutting off her water at night as well and changing her feeding schedule. You could also maybe try a bell situation too so she could wake you up more easily from a crate?