As oldbalboy noted, each regulatory authority establishes their own requirements in relation to the restraint of lap-held infants. With regards to the use of the infant loop belts, while a number of aviation authorities still require its use, others have chosen to no longer permit its use due to testing and research related to the hazard it poses. One example of such a report is from Germany:
For the transportation of infants on an adult's lap, the adult is restrained with a pelvic belt, and the infant is fixed on the adult's lap with an additional loop belt.
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In a suddenly occurring deceleration in the longitudinal aircraft axis, the adult and the infant show a pronounced jack-knife effect. The upper torso and the lower extremities of the infant as well as of the adult sitting behind the infant fold up in a forward direction, with the loop belt restraining the infant. Finally, the loop belt drives into the infant's abdomen and only stops at his or her vertebral spine. From the technical point of view, the infant acts like an energy absorption element for the adult; the crash loads acting on the adult are thus reduced, and the infant fixed with the loop belt thus suffers most serious up to fatal injuries.
Examination on the Enhancement of Cabin Safety for Infants
Of course, the safest route to take is to purchase a seat for your infant and have them secured in an approved child restraint system, not on your lap. One other alternative that you might also be aware of is the
AmSafe CARES device that is now available for use. Quite a few authorities have indicated that they will accept its use on board, including the
UK CAA.