I find that adding a child/infant (in my case 11 month old) will always add an arrangement that includes an additional bed even though in his case we won’t need it and as you said will increase the price.
I normally just book two adults and take the portacot and let the reception know what happened which is never a problem.
noted your child is older but would assume booking a room with 2 double beds (2 adults staying in room) and excluding child would be fine