hello dr....this is in my 6 year old girl's leg....now what is the reason behind this...and what is the remedy for this

A. Puss is there it is just a pimple that is there it’s a minor infection so if you see that it has made a mouth which I can see is there so just press it and punch it after that you can clean it with betaine liquid and you can leave it open it is going to heal on its own over the period and check with your doctor if needed