It's not racist. Nothing about them is racist. Children calling other children Zwarte Piet are racist, not the thing they are being called. It's like going up to someone from Kenya and saying "You're a Zimbabwean" which apparently means Zimbabweans are racist. Calling someone from Kenya a Zimbabwean isn't racist - it's ignorant, racisms forgotten cousin twice removed. It simply doesn't make sense and if anything supports the argument that people really don't know what racism is and use it lazily as an excuse without really understanding what it is.
If anything it's out of date, but that's about it. Giving children gifts isn't a social negative. If the Zwarte Piets were rushing down chimneys having sex with the children insinuating they're child rapists then it would be racist as it is giving a negative image of black people - suggesting white people obey the law and aren't child rapists. But no, they're doing something positive. It's like going up to a black person who is good at basketball and saying "Why are you people good at basketball?" The intentions behind the question are positive - the only socially inept people to get offended are those who really shouldn't be let outside for fear of being offended or people trying to make a case out of nothing.
So in short - no they're not racist. If anything they're out of date. The intentions are good - they are Santa's helpers spreading joy to children of all race, creed and background. Nothing is wrong with that.