If she is not old enough to be spayed, she is probably not old enough to go into heat yet. Most females go into heat for the first time at about 6-8 months. If you can get her spayed before then, you won't have to worry. Basically, if she is under 6 months, you don't need to worry. Females can only get pregnant when they are in heat and there is a small window when they are fertile.
If she does go into heat before you can get her spayed, you need to keep her and the male separate. A crate in your bed room will work. When you need to take her out, make sure the male is not around and she is on leash. Never leave her unsupervised unless she is in a crate and can't get out and other dogs can't get in. The doggie diapers are more to contain the mess than to prevent breeding. As I don't think you are planing on keeping her intact, a pair of underwear with a pad will work. Bigger dogs can use boxer shorts and a pad.
You should do a bit of research on dog breeding so you know what to look out for and prevent it. Knowing when she can get pregnant can help you prevent it from happening. You could contact a breeder about how they keep the females in heat away from the males.