Baby Copperhead snakes are poisonous just like the adult Copperhead Snakes so yes baby Copperheads are deadly if you get bitten by them and don't get medical treatment for the baby Copperhead bite.
Although a Baby Copperheads venom is relatively mild when it's compared to an adult Copperhead snakes Venom it's still poisonous and requires medical treatment to get rid of the venom so that it doesn't kill you.
If you do get bitten by a baby Copperhead snake you usually have a bit more time to seek medical treatment but you should still get medical treatment right away and don't wait too long.
A baby Copperhead snakes bite is usually more deadly to children than adults so be very careful around baby Copperhead snakes or any other snakes to avoid getting bitten.