Mosquito bites on toddler

Hi mummies, how do you prevent your toddler from scratching mosquito bites? Everytime my toddler gets mosquito bites it will swell until very big and she will keep on scratching until it bleeds. Have tried putting plaster but she will just tear it off in less than 5mins. Appreciate your answers! Also, do you use any repellent and is it useful?