My eldest son has a Hoy Bonaly 16" but we've always suffered with very stiff brakes. (https://www.evanscycles.com/hoy-bonaly-16-inch-kids-bike-EV203129)
It has Tektro RX1 levers I've tried loosening the spring tension in the calliper which made only minor difference.
I suspect my next step would be to lube the cables and see if that helps. There are no kinks or tight bends in the cable run so is there anything else I can do to ease these off?