Hi!
I realize I'm a little bit late on this, but I wanted to add my two cents in too!
I'm a Photographer specializing in Maternity, Newborn, Family and Child Photography....plus I have an almost 5 year old little girl (so I have a bit of experience in this).
First off, going to a Chain Studio with children that age is always going to be a bit hectic. Chain Studio's are often under major time constraints....they have a set amount of time to work to get the pictures and THAT'S IT.
A Professional Photographer may cost a bit more, but they will always be able to spend the time it takes to get those great quality shots of your children.
Also, you mentioned getting them to cooperate for poses. At that age children REALLY do better with On-Location Sessions at a Park. You can really capture their little personalities and see their TRUE self...not just a studio pose.
I personally always try to play and talk with the children for a bit first....then once they are more comfortable with me I'll start taking pictures.
I hope that helps, if you have any questions please feel free to email me at ____@____.com