I would divide the 10 gallon three ways, or get a smaller tank for the baby betta. Depending on the breeding net, its probably not a good size for a betta to live in permanently.
At this point, I think the issue wouldn't be if they bred if she escaped, it would be which betta would kill her if she did. If she's still a baby the bigger bettas may try to kill her.
Bettas are definitely an addiction!
I haven't bred bettas before. I seriously considered it a while back, but decided (and I think made a very wise decision) that I wasn't ready experience wise. A lot of things can go wrong with breeding. There is a great sticky in the breeding section that has questions you should ask yourself before breeding. It lets you know whether you may be ready or not.
I did a lot of research on breeding bettas, but decided I wasn't ready. Maybe sometime in the future I will be breeding bettas, but not yet. :)