could I get away with this?

I use FF miles to get a domestic roundtrip award ticket on Delta for 12 yo daughter.

a. 12yo flies to grandma's house (I accompany on the company's dime).

b. 2 weeks later, me, the wife, and 5yo daughter drive to grandma's house and stay for a couple of days. Then, we grab 12yo daughter, take her back home in the car, and leave 5yo daughter with grandma for two weeks.

c. 2 weeks later, I fly to grandma's for company business, then return with 5yo daughter on plane using 12yo daughter's RETURN ticket from roundtrip a. above.

Question: If the 12yo's name is on the ticket, can the 5yo use the return leg? No need for ID, right? Is this possible? Is it ethical/unethical?