Absolutely no risk at all for the children. Don't worry.
Flocoumafen (anti-coagulant in Storm and other rat-baits), used as a rodenticide usually supplied in waxed bait blocks, is insoluble in water, and the blocks are designed and formulated not to crumble.
I haven't seen the granules. I liked the idea of the blocks with the anchor-hole through the centre to save them getting dragged around.It also comes in granulated (pellets) form ...