Yes, it can be toxic to children, particularly through mucosal membranes. Here's the MSDS from JT Baker -- this is the same stuff as powdered borax, just called by its proper chemical name:
It does not break down in the environment, so I'd suggest a different method of flea control unless you intend to leave her out of the house +/- permanently.
Personally, I'd put Advantage or similar spot-treatment on the dogs and an IGR like Nylar down in the house. IGRs are extremely low toxicity to humans, but prevent baby fleas (and roaches and a number of other pest insect species) from growing up to be reproductive adults. I use a concentrate that's mixed with plain water and sprayed on baseboards, rugs, etc.
every 6 months -- $8-10 to treat 1500 sq ft if you buy it by the ounce container at a farm store.