Ay, I feel you on this one, mosquito bites on babies are such a hassle, lalo na pag nag-scratch sila at nag-darken pa! Honestly, a few of us here have found na the best approach is prevention muna (mosquito net, long sleeves in the evening), pero for the marks that are already there, you might want to check with your baby's pediatrician kung safe na mag-use ng any lightening creams or treatments dahil sobrang sensitive ng baby skin. In the meantime, try keeping the area moisturized and protected from the sun, kasi sometimes the marks fade on their own over time, my pediatrician told me this works for most babies naman. Good luck, mama!