I don't see any issue with it, as long as you feed them grit too. I give my chicks all sorts of things, scratch and veggies included, from day one, and feed them fine gravel from my yard for grit. Those blocks seem to me to be basically scratch feed compressed into a block with some additives maybe...years ago, I raised chickens from start to finish on nothing but scratch. I got some chicks as a kid, then my parents took me to the feed store and asked the man there what we should feed the chicks, since we did not know...he sold us a bag of "chicken food" (aka scratch) and that is all they ever ate. No fancy layer food, and they laid big, often double yolk eggs daily, more eggs than I knew what to do with!
I don't know about giving the whole huge block to birds that young unless you think they can eat it all before they ruin it by sitting on it and pooping all over it, maybe break it into chunks or just scatter a few handfuls of some scratch or treats on the floor every day.