Can I een as a number and as a noun? Why the translation may be "a child" and not "one child" ?
No. The number one is "één", with the accents over the ee. They're pronounced differently. "Een" is only used as "a/an".
Please read my reply to Wicked_Starfish above with a Je/Jij explanation and a link for more info.
Jij is the emphasized version of je. Je is just you generally, but Jij is you specifically. Like YOU have this thing, but I don't. That's where you use jij. Je would just be saying you have the thing.
I think Jullie is supposed to be the plural you. Since J is pronounced like Y in Dutch, I associate jullie with y'all.
They are different conjugations of the verb. Sort of like the difference between am, is, and are in England.