Less people died when they gassed the theatre, but there were more children in this setting.
Part of the problem using gas, is that it settles to low points. In the theater, the front seats may have been considerably lower than the back seats. I have no idea what the actually layout was, or where the hostages were held, but the gas can be more concentrated in some areas vs others. They gymnasium may have been a better setting for gas, since it was likely more level. Still, as you say, there were children. Children tollerate gas much less.
I was surprised to hear that there were very young children in this latest event. Some as young as 4-6 months old. Perhaps those kids attended not so much for daycare, as just the wards of mothers attending first day ceremonies.
When infants are involved, it's almost impossible to use gas without mortal results in the infant age set.
Thanks for the comments.