I guess I have been lucky thus far. I have 2 kids, age 5 and [almost] 3. The worse I've had was my 3 year old has got into my freeze-dried bloodworm's [right after I was done feeding] and spilled them all over the floor (which made the room STINK for at least a week). Aside from that, nothing. I have a 20 gallon long sitting on my kitchen counter. It's an open-top design, easily accessed by the kids, if they so desired. My other tank I don't think they could reach, being that it's on a 36" stand and covered.
I also let my kids help me feed the fish on occasion and I feel that I've taught them well enough to know not to mess with anything fish-related unless Daddy's right there with them.
The only real precaution I take is that all of my fish supplies are stored in an unused kitchen cabinet. Mainly because my stand doesn't have a cabinet and secondly for the safety of both my kids and my fish.