I've had a crazy mix of ages as well and it can be tough even more so when the older ones are building a tower. Nothing better to a little one than running over and knocking it down but not funny to the one building it!
An idea is to get the little ones interested in a toy that they haven't seen in a while ( I rotate toys in the playroom so when I bring out a different one, it's like getting something new)
Your just going to have to keep telling the little ones not to step on things or get their attention to other things.
Who needs a gym membership when your running around after these ones!