Because life should be relaxed.
Some children are naturals in front of the camera, but the vast majority are not. Many are wary of a stranger pointing a camera at them and I have the utmost sympathy with them!
Before I take my camera out I put children at their ease finding out what makes them happy and comfortable. A favourite book to read or toy is often useful and I will read with them, chat or play until they are ready.
I then let the children lead the session, watching what they do and occasionally steering their play in a direction that will lend itself to a better photograph. I may also ask them to pose to capture some more formal shots and sometimes just to redirect them a little bit.
The session will last about an hour, sometimes a little longer depending on how many children there are to photograph, and it will be relaxed.
Because life should be relaxed.