I'm a single dad of a 4 & 5 year old. The mother was in the wrong IMO. First in a place like that I would have a more watchful eye on my kids because the nature of the park. Its not like a normal playground where you can give younger kids some extra freedom and space. It looked like a clear accident. Now if the skater was acting like a dick or didn't seem genuinely sad that he knocked the kid down maybe I could see getting angry on him. But looks clear to me he did not intend to run into the kid and did feel bad, he seemed in shock/shook him self not sure how to respond.
The skater shouldn't sue anyone, though. Its not like he got seriously hurt. The mother overacted and got emotional about her kid. I know in certain situations (not this) if I feel a person has/could hurt my kid or is being a dick in some way to them, etc then they are going to feel parental wrath/instinct kick in. I have will not hesitant to lay someone out. When that instinct kicks in and what lines need to be crossed varies from person to person which this mother clearly demonstrates. Her actions are almost instinctual for the threshold point she has in relation to protecting her children. I'm not saying its ok, but i understand the process that's happening.