Wow this thread is sad. Why are people so invested in telling the OP that it is definitely not racist? Some comments are quite nasty where they could just state their own experience and put their opinion in a nicer way.
I don't have children but feel that if the OP feels like it may have involved some sort of racial prejudice or stereotyping then perhaps it did. I used to have people approaching me all the time to ask "where am I from", "where am I from originally" " yes but your parents are from X arent they?" etc. Sometimes it was ignorance, sometimes it was meant with bad intent/racially charged. Sometimes, if I feel the person is actually approaching in a nice manner, I answer them how they want answered. Other times I choose to say I'm from "X UK town" and leave it at that. You have no idea how many times people have gotten aggressive with me, shouted in my face etc. when I haven't answered the way they wanted.
Just because white women have been asked the same/similar question does not mean that when the OP was asked it was not racial. I suppose each asker would have their own intent no? With my example above though, I definitely felt that there was some sort of bad/racial intent - and most often that was proved later on in the conversation.
People keep saying that she has used the word "coloured" and that makes her racist. Well it really depends on your background or nationality. There are many ex-colonial countries where people still today use that word to describe themselves and others. I'm from the UK but my parents used this word as it was how they grew up describing themselves. I had no idea that some people found it offensive until I was at high school. Also, if you are describing yourself, are you not allowed to use any word you like?
People are also saying that it's not common for a dark skinned person to have a light skinned child? This seems really strange to me! A large percentage of my family are dark skinned parents with light skinned children - we all live in the UK. I understand that some may not be aware, but everyone has the capability of being polite.
I feel that it may be rude and ignorant, sometimes may be racist. It's definitely entitled though, to just walk up to someone and ask them questions like these based on the colour of their skin. Plenty of other topics for small talk. I always thought people striding across shops to ask my background and demanding that I answer them correctly was entitled.
It also seems that people don't actually look at the features of the child, just the colour of their skin and hair. One of my family members is dark skinned and her child is light. They have exactly the same face though, expressions etc. so in my opinion the child takes after the mother, rather than the white father.
Sorry this was long....