Delta used to pay for ground transportation between LGA and JFK on inter-continental itineraries. The international conditions of carriage used to provide for this explicitly; now they do not, although the current language is somewhat intriguing:

"Although Delta may assist you in making arrangements for such ground transfer service, you agree that Delta is not liable for any act or omission of any such independent operator. If Delta issues you a voucher to cover the cost of services provided by any such independent contractor and you decide not to use the services, the voucher for such services will have no refund or cash value."

(Emphasis added )

I would at least ask at the DL Baggage Services office at LGA about this. Note that the passengers are responsible for retrieving any checked baggage at LGA, and re-checking it in at JFK.

Better yet: I would try to get the MCO-LGA flight switched to an MCO-JFK flight. If the travel agent cannot do it, I would at least ask at the DL check-in counter at MCO. Kind of surprising that the travel agent did not book them on MCO-JFK-GVA in the first place!