Baby
noun
ちゃん is the honorific you put on a child's name. So, who is this mysterious child you're calling red? When a baby is born, what color are they? You got it, they're all red. And, because you want to be nice and cute you add ちゃん to that. That's why あかちゃん is a baby.
あかちゃん
If you know the reading of the kanji 赤 you should know the reading of this too.
五月に赤ちゃんが生まれる。
A baby will be born in May.
このかわいい女の子の赤ちゃんはエマちゃんです。
This cute baby girl is Emma.
これは赤ちゃんのミルクです。
This is baby formula.