It's normal to have doubts, especially when you're making such a big change to go back into education. But it might help to realise that you don't have to decide just yet, and that it's not impossible to change your mind. You might just be wanting the other course because you think you can't have it, and the grass is always greener.
But it should be possible to change course at the same uni if you ask them. The course might say it's closed but that might just be because they don't want the flood of new applicants they otherwise would get. Alternatively, you could ask the uni to reject you if you wanted to try [wiki]UCAS_Extra[/wiki], or you can wait a while to use Clearing. Or there is of course the option to take a year out and reapply.
It might help to get some work experience. You say that you have children, so would you be able to help out at their school? Even if they're the wrong age for what you want to do it could help, or just ask any other school anyway. Criminology and social work would be a bit harder to arrange, but you could possibly arrange something with a care home which is similar.