When an I-140 downgrade to EB-3 has been filed as an amendment instead of a new case, to get back into the EB-2 category, you have to file a new I-140 using the same PERM application. If the dates are current, you can concurrently file for adjustment of status.
I am not aware of any law that prohibits attending H-4 visa stamping before H-1 visa stamping. Your spouse has an H-1B approval. But the consulate can ask you to wait till the H-1B visa approval. Email the consulate to check how they would want you to approach this.
H-4 visa stamping does not require any H-4 approval from the USCIS. The H-4 visa is approved based upon the H-1B approval.
Check the website of the US consulate where you're going for the procedures. To the best of my remembrance, children under 14 are not required to be interviewed.