That's incorrect for two reasons. First of all you need the instrumental case 'dzieckiem' here.
Secondly, while the sentence 'Jesteś tym dzieckiem' is correct, it translates to 'You are that child', which has a different meaning from 'You are a child'.
Theoretically you can say "Ty to dziecko" if you want to use nominative. There's no verb in this sentence although. But it sounds unnaturally in Polish and I don't think there's even any english equivalent of this construction. ;)
Someone else wrote in the Polish Duolingo that "to," cannot be properly used with first and second person singular and plural perspectives ja, my, ty, i wy. That may be why it sounds unnaturally in Polish.
Frankly the word "dzieciątko" really sounds to me as if you were talking about Baby Jesus (although it would be capitalized then). It probably can be used normally, but at least to me using it it would be a bit surprising.
But it's not good to use the construction with "to" if the sentence starts with a personal pronoun. A noun, a noun phrase - yes of course, but rather not a pronoun. So avoid "On to dziecko" and instead use only "On jest dzieckiem".