Given the way they all died, I would suggest the carpet or something else in the brooder introduced a toxin or parasite. Peas are hard, but not that hard.
We rarely lose peas after the first few days. All of them are on medicated starter. None of our peas really gets grit until they hit the ground and they are getting pellet food before they hit the ground. I think your problem lies elsewhere. I don't think it necessarily hurt them but we are HUGE proponents of no food but chick starter or layer pellets until three or four months. They may get a freshly cut watermelon or cantaloupe but that is it. NO scratch or other foods. I don't want any possible contamination, worms or parasites. The wife and daughter frequently violate this rule with well scrambled eggs. Our theory is why introduce possible issues during the critical age?