I’m sure the fiancé visa is to get married within 6months.
If this doesn’t happen during this period and she doesn’t want to settle in UK I’m sure there is no force to this. She can go back easily. I think the transition to UK life is easier without prior children in the relationship. But this is just my opinion. 
I don’t think the success rate is great tbh. She needs to own land have have a decent job to return to. These show she a tourist in UK. Or you can apply for a fiancé visa which might be easier.