anoxia? in an open and ventilated area? i don't think so. Do you know how much CO2 it would take to cause total o2 depravation even in an enclosed area? there's not even a risk of hypoxia here. the co2 will hug the surface of the pool, but A - there's not enough of it to cause a significant (read: dangerous) spike in CO2 and more importantly B - it is easily dispersed with normal air flow, kinetic energy and movement, such as one of the kids popping up out of the water.
i agree about the bottles. although even then...is it that much more dangerous than ordinary fireworks?