You need to get over your weight issue. Kids have been lugging 30lb bikes since kids were riding bikes. Weight is important but its not a huge deal since kids blow past the 24in size too fast. Remember its near their teenage years. Also when we're talking weight, dropping sprung weight or mass isn't nearly as important as unsprung weight/mass.
Obviously it depends on the actual riding but ...I think it depends on the riding you are doing. Chairlifting up and riding down, weight might not matter so much.
Using the same bike to climb? 4-5 pounds makes a huge difference to me, and I am a full grown 165 pound adult. Now imagine your 80 pound kid. My 31.5 pound 150mm travel bike sucks compared to my 120mm trail bike that weights 27 pounds.
Kids shouldn't have to suffer on heavy bikes for 2-3 years because they outgrow them so fast. Also, define fast. My son was on a 24" for about 3 years on 24" wheel size. He developed some pretty awesome riding skills and rode some distances many adults still talk about - I'm not so sure either would have happened if he was on a 28-30 pound bike. YMMV.
Remember its near their teenage years. My kid got his 1st 24 at 6 .... whatever way you look at that its a long ways off teenage years.
Either way ... I agree I can feel the difference climbing of 4-5 lbs and different geo... but I think a lot of this depends how fit the kid is and practice. My now 8yr old's FS is 4-5lbs heavier than his XC bike... and yes it makes a difference especially either racing XC or 3000' climbs on a 20 mile day....HOWEVER .. he can still do that 3000' climb in the FS... but it takes longer. HOWEVER ... I honestly believe he'd never have got to that stage starting off on the heavier bike... the fact he has the light XC and he was able to get into both the specific physical fitness and mental place where he knows it's possible is largely (I believe) down to starting doing this on a 20lb bike.
When I stick on the DH wheels & tires the bike is WAY heavier... but also draggier and its all rotating weight... there is a much bigger difference adding the wheel weight to the point we go far slower and far less distance.