What is the ruling on interrupting the tawaf due to severe crowding and fear for young children, and what are the consequences of that?

If the tawaf (definition) (circumambulation) was a nafila (supererogatory act), then there is no issue in interrupting it, because it is not obligatory to complete acts once begun, except for Hajj and Umrah (definition).

However, if the tawaf was obligatory (like the tawaf of Umrah), and it has been completed, then there is nothing upon you. But if you have not returned to it, then you are still in a state of ihram (definition) (ritual consecration), and you must refrain from the prohibitions of ihram, and head to Mecca to complete the Umrah.

As for committing prohibitions of ihram out of ignorance, there is nothing in it. But if it was done knowingly, then a fidyah (definition) (expiatory offering) is obligatory for each prohibition, except for sexual intercourse, which invalidates the Umrah and necessitates its completion and a make-up (qada').

It is worth noting that dedicating the twenty-seventh of Ramadan (definition) for Umrah has no specific evidence for its merit.

As for children, there is nothing upon them if they do not complete the Umrah. And whoever is unable to return to Mecca is considered muhsar (prevented from completing pilgrimage) and is permitted to exit ihram, and a fidyah is incumbent upon him.