The M80's are pretty heavy and would take a lot force to knock over. However with small children (I have 2, a 4 year old and a 2 year old) never say never.
Murphy's Law prevails!!!
You need to watch them (speakers and kids) like a hawk, keep grills on at all times. If the kids don't know the grills come off, it's easier. If kids get within 200', um.... I mean 2' of speaker explain to them in a very loudly (this shows you mean business) that speakers are worth more than they are and it’s cheaper to ship kids than M80’s.
If that doesn't work you could always suddenly and quickly turn up the volume to 110db SPL when they are standing too close and literally scare the ____ out of them. That should make them afraid of said speakers and keep them away in the future.
Although this method is very effective and depending on the amp you use, you may need to check speaker for damaged tweeters. This of course defeats the purpose of having the kids near the speakers in the first place. So be very careful!!!
