Some of the responses on this thread are utterly delusional.
Once again, despite the name of the site, I am convinced half of the commenters have either never had children, or it's been so long since theirs were little that they can't remember what it was like.
Yes, there are plenty of calm and quiet activities that will occupy a pre-schooler - for 30 minutes max!! And then they're up again. Even something "calm" like baking cakes with my LO involves her trotting around here there and everywhere all over the kitchen. My 3-year-old even stands and walks around while watching telly. Most pre-schoolers physically can't stay still for more than half an hour from the minute they wake up. Ditto with taking them outside... yeah, great - for 3 or 4 hours max. What about the other 8-9 hours they are awake, when you need to get housework done, get their lunch ready etc., and meanwhile the children are walking up and down.
And very young children naturally walk with a flat-footed gait. That's part of their physical development, it's not something you can or should stop.
There was a court case about this very topic in Germany a few years ago, and the German court ruled that children have a right to play, and therefore that neighbours had no right to complain about ordinary levels of noise made by children playing in their homes during the day.
Whilst of course you can discourage things like jumping off the furniture, screaming (not saying you allow that anyway), etc., and you could get those foam mats and a few more rugs, your neighbour can also buy some noise cancelling headphones, or, failing that, rent a workspace.