If those are grass shrimp (and I'm not good enough to tell by looking) then all you will need to do is figure out a way that they don't become food. Many breeding nets are designed so that fry will pass through, but adults can't. I'm not worried about your shrimp eating their own young, but I'm worried about the Betta doing so, a net then, would be a bad idea. Or, if you got one, put the betta in it not the shrimp.
Your best bet, imho would be to have a seperate breeding tank for your shrimp. It doesn't have to be huge, a 1 gallon jug would even work. (You know all those adorable tanks that you've always wanted but are cruel to keep a betta in due to their size? Now you have an excuse to get one.) Large "Critter Keeper" boxes also work well. I would throw in as much plantlife as you can. If you have an extra sponge or box filter, I would put that in the breeding tank too.
You may need a THIRD tank too, in it you will raise brine shrimp (sea monkeys) according to the standard sea monkey directions. Here is the tricky part: you have to time your births so that you have near newborn brine shrimp at the same time your grass shrimp hatch. Baby grass shrimp are tiny, and there are very few things that they can eat, baby brine shrimp being one of the few.
Anyone know what else to feed baby grass shrimp?
Also, most of what I'm getting I'm more or less paraphrasing from shrimp-specific forums and pages. A google search for "Breeding ghost shrimp" or "breeding grass shrimp" may give you more information.
