I don't have any lighter suggestions I'm afraid.. I just remember feeling quite dejected yesterday as I lifted yunki jr's £200 Islabike into the car, swiftly followed by his cousin's £40 bso.
If you look a the numbers, they are surprisingly lighter. All the cheaper ones appear to be at least 50% heavier, like the £240 trek mt60 weighs something like 6kg more than the equivalent £300 Islabikes, the Carrera Blast weights 6-8kg more depending on which weight you believe. The Decathlon ones are roughly the same weight as those two, so probably the closest to Islabikes weight at Decathlon prices is the Decathlon bikes assuming you're buying new; they aren't that heavy for kids bikes.
With the even smaller bikes it is just as bad, I did tons of research when buying Rose's 14" one and the only one close in weight was the Frog, which was a fiver cheaper. You can pay £150 for a Ridgeback, but even that was 50% heavier.