I have flown SQ Y+ and have flown with a child under 3 but in those days CARES harness were not a thing or possibly even around. I did a quick Google search and looking at the images the top seat strap above the child's shoulders just seems to loop around the seat back and tightened. Looking at the images most of the photos are taken in Y, Y+ seats on SQ are very similar to Y with a bit more leg room so there should be no issues. Final thought if these harness are now required the flight crew should know how to fit them and should be able to assist.