It is very important that both parents and healthcare workers responsible for a child, can show that the child is developing correctly and learning new skills suitable for their age.
Certainly when my daughter was small, as well as having regular inoculations and checks by the local nurse and doctor, we also had a healthcare professional assigned to young children who judged her growth rate both physically and mentally.
She was asked to draw pictures which (I am guessing) had some sort of significant meaning to them, and they also asked her questions. Children that age (around 2 or 3) will say anything when asked so she was always honest and open in a way that only children are.
My daughter luckily developed perfectly fine, but there were other toddlers in the village who required speech therapy or some further help with learning motor skills etc. Most of these children caught up with their peers within a year or so because problems were found early and dealt with.