my baby is 1year 9 month old they rub their private part and it become very hard on that time why this is happened ?

A. Even a small child is a constantly keep rubbing it it gets little hard it’s that tendency of that area but why the child is doing it that you really need to understand because some kids just get into the habit of doing it gently without understanding anything and sometimes kids have some irritation or eating which is either behave like this so you should try to figure out where it is going wrong