Hi,
The safety of oxybutynin in children under 5 years has not been established. You can read about the effects of the drug at the link pasted below.
www.rxmed.com/.../b2.1.monographs/CPS-%20Monographs/CPS-%20(General%20Monographs-%20D)/DITROPAN.html
Urodynamic studies have been performed even in infants, children less than 1 year of age. However, children younger than 5 years have been considered unsuitable for invasive urodynamics as they are usually uncooperative and may require general anesthesia during the procedure.
The child's urologist will be the right person to decide on this and he/she will also refer the child to an anesthesiologist for an evaluation regarding the safety of any anesthetic procedure.
As you have not mentioned your child's age, it is not possible to assess whether he can have any other problems with the procedure.
General anesthesia does carry some risk of complications and will need to be evaluated in view of your child's age.
Do keep us posted on any further doubts in this regard.
Pls respond to the question ASAP