We were at the St Ermins in March 2 adults and a child. I am platinum SPG and Marriott. At check in they 'upgraded' us to the smallest room you have ever seen - we actually could not get all our luggage in. So, I went to the front desk and asked for an upgrade to a bigger room or a suite. They offered me a suite upgrade for 180 euros a night on a 7 night stay. Made me angry - I know its not guarranteed in the Marriott program - but being used to it in SPG - it did not feel right. So, I declined and they gave us a larger room for free. We struggled through it for 4-5 nights of our stay until we could not take it anymore and I went back to the front desk. Luckily I got someone else who offered a suite upgrade for about 80 euros a night for the last two nights - so I took it. But, we won't be back because of the rooms.
It's a shame - the breakfast is awesome. The in house restaurant is great. The location was stellar and the staff (other than the front desk) were good as well.
My advice? If traveling their with a child and you are used to space or need it - make sure you book the room category you want ahead of time. This is not a place handing out upgrades without being paid for them.