Do children of Indian parents born in Thailand need a return flight ticket without a Thai visa?

Need an advise:

As my Indian friend is informed from immigration, there is no need of getting a visa for child up to 15 years old if the child is born in Thailand. Both the parents are Indian. Last year, family exited the country without any problem but when returning, the immigration asked for the child’s return ticket. So my friend needed to flight return ticket instantly and passed through the immigration.

Is this the normal process for a child without Thai visa to have return ticket?

Advantage of not getting visa:

No 90 days report, no TM30, no visa fee, no reentry required.

Disadvantage of not having visa:

Reentering thailand needs return flight tickets, and need to book one adult ticket as well.

If there is a way out without booking return flight ticket, it would be great. Thinking of booking a flight ticket via agent and cancel it!

Need an advise/suggestion from the experts in the group.