Welcome to FlyerTalk, Annelid! That connection time should be doable if the inbound flight is ontime. Are they booked on a single ticket or on two separate tickets? If the former, they will be protected in case of delays, but not in the latter case. Conservatively, I think they will probably need about 60 min to collect luggage at LGA, 15 min to get a taxi at the queue (ask the dispatcher for a taxi that will take a party of 4 with luggage), 15-30 min to travel to JFK, 30 min to check in and recheck bags, 30 min to clear security - all assuming no status. Others may have more advice about the timing and ways to make that transit.