I hate it when people ignore/speak down to children. My son doesn't so much have this problem in my home town because it's a very small place and every one knows him. Elsewhere though, oh boy. He's six and very independent, but he's also very small for his age (nothing medically wrong, he's just tiny. He has three year old friends the same size as he is). He's quite capable of buying small items for himself if I give him the correct change and he loves doing it and feeling like a 'grown up'. Many, many times he's stood at counters being completely ignored while he says, "excuse me," over and over. I step in eventually, but it's frustrating for both of us.
I've also had people block him from leaving shops to go to the car ahead of me and completely ignore him saying, "it's ok, Mum said I could." I had to literally bite my tongue when one woman physically grabbed him.