My baby has this dark marks/ scars from insect bites. How can I lighten them?

As baby's skin generates and renews fast, the mark will disappear after few weeks. For me I did nothing about it and the marks faded itself. If you do not want the mark after each mosquito bite, you can teach your kid not to scratch them (lightly pat on it instead) and apply mosquito cream e.g. calendula remedy.