Really, I think that airlines should have some defined adult/child ratio (and no, a 10 year old sister should not be classed as an adult). What if there was turbulence, and the toddlers panicked? Who would make sure they stayed belted in? What if, heaven forbid, they had to evacuate the plane? It's hard enough for adults to do that unencumbered. No way someone could take more than two toddlers with them.