Hello Lotte42! I think you'll find that most artists have their own "formula" for skin colours, whether a childs or adult. My favourite mix for watercolour skin tones is straight Burnt Sienna, obviously at the correct strength to give the right tone, and into this I use brush tips of Permanent Rose (to warm the shade) or Aurelin (to correct).
This is not everyones idea of achieving a skin tone, but it works for me.
Have a go and let me know how you go on, or if you tried anything different.