The Carrera Blast 20 is actually pretty light, the Blast 24 is a properly heavy lump though. No idea why such a big difference.
My 7 yr old has a Blast 20 and it's really brought his riding on. Bought it 2nd hand as he was indifferent to cycling at the time.
His older 9 yr old brother has always loved riding so bought him an Orbea MX24 Team which is seriously light and Deore mech, shifter etc. It's like a smaller quality adult bike. Weighs under 20lbs I'd guess.
Plan is that the 7 yr old will inherit the Orbea when his brother moves to 26".