Sadly, energy expenditure seems to correlate inversely with what's required. When I was a kid I was happy to sit and watch cartoons all day, requiring almost zero energy. But now that I need it...
A massive amount of energy is just used by the developing brains, so you are still using that as you watch a cartoon.
"The growing brain is likely the key energy sucker in little kids, says biological anthropologist Chris Kuzawa of Northwestern University. Kuzawa did not participate in this study, but in 2014 his team found that the brains of young children consume a stunning 43% of all energy used by the body."[1]