Hello and welcome to HCM.
Thank you for your query.
Oily urine or in your words an oily slick in the urine is most probably due to the presence of ketones in the urine. The appearance of ketones in the urine are due to:
Other causes can be:
disorders (eg. early jaundice
If you continue to see this oily nature of urine in your child and if he is not on any medication, I would advice you to take him to visit his doctor at the earliest for an elaborate examination, as all conditions in a child can be aggravated and more pronounced.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications. I would gladly help you.