I'm not 100% certain about WDW reservations but with
Disneyland and Aulani (from experience), all guests including infants are counted towards room occupancy. A friend of ours told the
travel agent when booking their trip to WDW that they had 2 adults and 3 children (9, 8, 4) but failed to mention their 1 1/2 year old so when they arrived they had to be switched to a suite because they had 6 guests. I'm not sure why people think when asked how many children they have, they don't think to mention babies or toddlers lol. But you're correct in that if they can't be accommodated with a daybed then they might be able to request a cot.