Prelim calculation of my tax returns, I used a hypothetical ITIN/SSN for my kid (3XXX). When I updated with the actual ITIN (9XXX) the return dropped a lot. Any idea why?

Certain child tax credits are only allowed with an SSN (citizen or permanent resident).  ITINs and SSNs follow particular numbering patterns and TurboTax knows the difference.  

So, you are saying that if we use a number that starts with 3 turbo tax is doing one calculation, if we use a number starting with 9 the system does another? Is that correct? What are the differentiation? Which credits are considered for a SSN that is not considered for an ITIN?

You cannot get the child tax credit unless the child has a SSN. If the child has an ITIN you get the $500 credit for other dependents instead of child tax credit.

Thanks. That's clarifies my doubt! Have a great day!