[gemy]

I know your question was about trying to stop your females from "humping", during their first heat. You can try to redirect their behaviour, just like you do for any other behaviour that you don't want. But it is something females will do, and in fact males as well who are sexually intact.

I'm going to assume (and I could be wrong), that you are relatively new to owning dogs. So you must keep your babies safe from any male dogs during their heat. No letting them run off lead. Walk them only on lead. They are way too young to mate, and that is a road you don't want to go down.

For females and their health, research indicates, that the health benefits of spaying, likely exceed the health risks of not spaying. If you continue with your decision to not spay, you will need to learn the early warning signs of pyrometra, and mammary tumors. Speak to your vet about this. Also you can go to any pet medical advice site, and find out.
One of the problems with pyrometra is that often there are no visible early warning signs. Literally as one poster said, your dog can seem fine in the AM, but by the time you notice something is wrong and take her to the vet, it is already too late, the infection has spread, the fever is out of control, and your dog dies. This can happen in a 24hr time period.
Mammary cancer, has a high metatasis rate, although again a bit better with early detection signs being present. Talk with your vet, how often and what to look for in your dog(s) for signs of this.

Seriously consider a low cost spay and neuter clinic. If they are too far away from you, then budget for the spay. Start putting away 20 or 25 dollars a month (you can buy Canada Savings Bonds at any bank) and they will make monthly automatic withdrawals from your account. Easy as pie. In two years you will have enough funds to cover one spay, and so on. Or make the amount larger say $50 and in a year you are good to go with a spay.

Are you in Newfoundland or Labrador? I had one of the best vacations of my life in St John's Nfld.