The Graco collapsible playpens should work in most V-berths of 30 feet and over sailboats.
The kid might sleep in the cockpit lashed into a car seat. Mine did until about a year old.
Tell the parents a fitted PFD. Some kids are huge, others aren't. The kid must wear one, even below, because they can't save themselves if the boat goes under by climbing out, and they frequently fall over. A fat foam PFD can and does cushion the blow.
Put 'em on a halyard if in the cockpit. They can't go on deck at that age, because they could easily fall...even at dock...under the lifelines and onto a bollard.