Children are usually immediate-or-cancel takes or short-lived passive posts. They carry their own client-order-id, appear individually in the execution-report stream, and are the only thing the market ever sees.
Child sizing matters. Too large and each slice moves the price; too small and you pay more in fees, messaging and queue-position resets than you save.
Example: a 100,000-share parent sliced into 200 children of 500 shares. If each child costs a tenth of a cent in fees, that is $100 total, trivial. If each one also loses a penny of market-impact, that is $1,000 — the real cost is impact, not commission.
Related: parent-order, order-slicing, algorithmic-order