in this case, just book for 3 and request a cot. but i' surpriced at Dubai - we were there many times with 2 kids (room for 4 ) and I never had problem booking 4 people normally. this was for Westin Mina Seyahi, Le Meridien Mina Seyahi and I remember checking out a few other marriott properties and it wasn't a problem either. Could be that all double queen rooms are taken and only king rooms are available?