YANBU - I agree that it's totally normal toddler behaviour, but at 6am (which is a noraml time for a toddler to be awake), if I were his parents I'm afraid I would be a bit more considerate to other people in the flats and just not let him run up and down at that time! Or if that proved too difficult, then take on board her complaint and sort out the soundproofing/rugs.
Why are all these people saying "It's just what toddlers do". Well, I'm sorry but there are LOADS of things that toddlers would do given the chance, but the point is that as parents WE are the ones who are teaching them to live according to societal expectations and niceties - one of which is you show respect to your neighbours at a time like 6 in the morning!
I bet the parents go back to bed and let him run round on his own instead of getting up with him and maybe doing a quieter activity with him until a slightly more decent time.
I was livid once when DH and I were at a hotel one Valentines night and we'd been given the bridal suite. All romantic etc and an expensive treat (seems naff now, that Valentines stuff but anyway, it was early days then so highly expected
). We were woken up at about 5.30am by some kids upstairs running up and down for about an hour and a half and screaming. Put paid to any morning shenanigans, I can tell you, I got so wound up. Maybe we should have deliberately had really loud, embarrassing screamy sex - that would have shut the buggers up, bet their parents would have taken them out 
Poor OP, YANBU! I have lived in flats, and have had 2 energetic toddlers, but I know how to treat people with respect and consideration, unlike some people on here! I am amazed.....