1.9 oz urethane is usually very waterproof, so could be made into lots of things. Bivy floors of course, but also rain mits, rain chaps, poncho-tarps, etc. The extra weight over silnylon is not that significant. Especially if you are making custom sized gear for kids.
Oh, and poly ripstop will usually not stretch when wet like silnylon, so tarps will stay taunt in the rain. Even if tarp/bivy camping is not your thing, a tarp can be nice to have for hanging out in the rain, or keeping gear dry without having it in a tent.
If nothing else, you could sell it on the Gear Swap.