Hello and welcome to the forum.
I am sorry to hear that your child is having some discomfort in his leg. As an adult cardiologist, however, I do not think I am in the ideal position to be providing you with medical opinions as it relates to your 3 year old child.
I would suggest you either post your question in a pediatric cardiology forum or, better yet, given that your child's operation was only one week ago, call the doctors/nurses involved in his care to discuss this.
Thanks.
Dr. Rich