tea tree oil
Apply straight on the affected area (if sensitive, dilute in a carrier oil such as olive, coconut, almond, jojoba...).
In my (children's) experience, the itchiness goes away almost immediately or never even starts if applied early. It also heals faster.
May need to re-apply one more time if itchiness returns, but that's usually it.
*To prevent bites, try this Anti Mosquito Spray