My experience with flying with non-US airlines (who are not under the FAA mandate to accept CAREs) is that once you show to the startled FA that the kid is secured by the regular seatbelt and that the extra stuff is truly extra, they don't care as the relevant requirement -- that the passenger is held by the seatbelt -- is met. One particular FA even asked me for info about it so she could get it for one of her kids!