Wise words. My son has been doing the same watching the neighbors but in this case it is the kids on the soccer team:

" Dad, when are you gonna get a new car, this one is too small and old"
" I want new soccer spikes, "John B" has 8 pairs"

And it goes on. I should calmly tell him what your Grandma told you, though I do, but the key word is "calmly" .