Well the situation does need to be addressed.
But I think the way it is approached has a direct affect on the out come.
If you contact the landlord or the police I feel like it could escalate the situation more then might be necessary. I feel like the down stairs neighbors will assume who called any way!
Try not to judge the book by its cover, he could be a good guy!
I think I would go downstairs and approach them as a friend who lives below you. SUGGEST or REQUEST that yall try to figure out a way to get through this in a civil way.
I would bring up the fact that you KNOW your child (a 2yr old) can get a bit active at times. But reassure them that you stay on her about her "inside feet". Letting him know you are trying to keep from disturbing them.
Then bring up how there are times when their music disturbs you and that you would hope that you guys can figure out a way to try to work things out so you both can enjoy the current arrangement.
If this doesn't work then things can get a bit tough.
But I would try to be neighborly first, then gather the lynch mob and contact the authorities!
Good luck and keep us updated!