A couple of years ago, my sister-in-law asked me to crochet my nephew (her son) a rainbow striped scarf. He was 3 years old at the time and she said that by the next winter he would start refusing rainbow colours, so she wanted him to have one while he was still young enough. They loved the scarf so I made him a couple of pairs of mittens.
The pattern for both the mittens and scarf is available in my etsy shop, and on Ravelry. To make them you'll need double knitting yarn in 7 colours and a 5.5mm hook. The mittens can be plain, striped or blocks of colour and will fit a 2-4 year old.