What does your plan say? Usually the parent beginning their time gets the kids from the other parent. This prevents the kids from having to wait on the other parent. This is not always the case so read you plan closely.
This answer is provided for general purposes only. If you need legal assistance you should consult with an attorney. Responding to questions DOES NOT create an attorney-client relationship.
Admittedly, the clause you referenced "on-residential parent will return the children to the custodial home on sunday 6pm" implies that your ex would be responsible for the transportation to drop off the children on Sunday evening; however, as the previous answer indicated, I'd fully review your court order, including the Shared Parenting Plan. One of the purposes of said plan is to account for questions like these, and they often include a section entitled "transportation". As the previous answer also indicated, it is more common for the picking up parent to provide transportation, to ensure that the children aren't simply waiting to be picked up, however, that's only the norm. I'd fully review your document, and possibly talk to your ex to set up an arrangement as to who should transport at what times.
This answer is provided for general purposes only. If you need legal assistance you should consult with an attorney. Responding to questions DOES NOT create an attorney-client relationship. Nor does this answer contain legal advice. This answer is simply designed to answer a question and any information gained therein should be taken as information and not advce.