Good question. I have a glass tank, and a son who will be three in November, so I need to think about this too. Acrylic is indestructible, but scratches easily, usually when sand gets on the "soft" pad you use to clean the inside, so be really careful. For 100% peace of mind, go with acrylic.
No kid will be able to break the glass without the use of some hard implement (stone or metal) that has some mass to it, so you don't have to worry about 99.9% of the horse play that kids do. Wooden and plastic toys are no danger. If a kid were to tap on the glass with a hammer, then you might have a problem, and some kids like to experiment. There's only a narrow window of time in which a kid is strong enough to swing a hammer, but clueless enough not to be able to predict the consequences of hitting the tank with it, and once that age is reached, the tank should be safe.
The vast majority of glass tank disasters seem to come from stands that aren't sufficiently flat or don't support the weight evenly. I've never heard of one being broken by accident (except when being moved), by a kid or an adult. I'm willing to take what I think is a tiny risk, in order to avoid the hassle of easilly scratched acrylic.