Bring a cloth and a little spray bottle of window cleaner.
Shoot as perpendicular to the glass as possible for less distortion. This is especially important with thicker glass and/or when shooting into an aquarium.
A rubber lens hood can be a big help as mentioned. Brace yourself against the glass to hold steady. Wash the window after you're done to get your hand prints off it if you're a greasy/oily sort of person (I am).
Some displays will be behind old plastic that's really scratched up- these can be a waste of time photography-wise.
On camera flash will be pretty useless, but if you can get the flash off camera with the help of an assistant you can get great reflection free results (not the most practical at a zoo, but doable with another pair of willing hands).
I'll also recommend shooting in RAW if you don't already. White Balance can be a nightmare in indoor exhibits, and RAW let's you adjust after the fact at no loss. This alone is worth it.
Don't be totally shy of reflections- the animal behind glass with your grand daughters reflection superimposed over it is a pretty overdone cliche, but when it's your own kids go for it.
Don't forget to have fun on this family outing