After spending quite some time looking for the Email Address of the Plaza Seoul hotel myself, I thought I would share it with the rest of the world.
You won't find it on the Marriott Website (why not actually?) and the Contact Us link located at the bottom of the Plaza Soul own website doesn't work either. Annoying and disappointing at the same time!
Anyway I found the Plaza Seoul contact form through Google https://www.hoteltheplaza.com/eng/etc/contactus.jsp but don't waste your time filling it out. I did and even though I got the confirmation that the message was delivered they never replied and also told me that my message never arrived!
So one address I got on the phone is: firstname.lastname@example.org and the other one I found is: email@example.com
In the end I recommend calling the Plaza Seoul hotel +82 2-771-2200 and to find someone who speaks proper English. One very helpful individual is the supervisor of the Lounge Mrs. Elli Park.
BTW, the Plaza Seoul is quite fussy when travelling with kids. So for instance they don't let kids younger than 13 into the executive lounge. I mean not at all. So if you think you can at least have breakfast in the lounge together... well you cannot.
The breakfast charge per child is 20.000 KRW. Outrageous if you consider that a 5 or 7 year old child will consume a bowl of corn flakes.
The only deal I was able to make on the phone was, I'll book a rate with breakfast to cover the kids and get the breakfast for the parents on my Marriott Gold benefits. The breakfast rate, which covers two adults, was 20.000 KRW more expensive that a rate without breakfast, i.e. 10.000 KRW per person. Otherwise 20.000 KRW per child, and 25.000 KRW per adult...
To be fair they at least upgraded me to a suite and got me a roll away bed for free. Otherwise the roll away bed for a little child costs extra too. For an adult ok, but for a 5-7 year old child? Not very family friendly.
The only reason I'm staying the the Plaza Seoul is the central location. Apart from that I prefer the JW Marriott Soul which at least will let you have breakfast at the lounge and when I asked them about a roll away bed suggested that I can a room with two double beds, which would save me money.
Also stay clear of the JW Marriott Dongdaemun Square Seoul which is very nice to stay for adults but fussy when you bring your kids along.