Questions for parents with small children that live in an apartment.?

So Saturday I just moved in to an apartment (from a house) with my 1 1/2 year old son and almost 4 year old son. They are SO loud. Not just yelling, but jumping, crawling etc. How do/did you keep them quiet? After talking to my almost 4 year old and reminding him often he's pretty good about it now, but my 1 1/2 year old there is no talking to him about being quiet. He doesn't yell much or anything but when he crawls it probably sounds like a stampede to the people downstairs. They came up once on the day I was moving everything in Saturday and asked me to keep it down. :-S But I can't help that my 1 year old crawls loudly. I guess they'll just have to deal with it? Not sure what the consequences could be to being loud. But it's not like I'm having a party. I have 2 small children.