The playroom itself I see in a different light, now having seen some of the episodes aagin. It's not that David and Hallie want to hang out there. At first they go to look for it as a lark, something teenagers would do, searching out sealed rooms and family secrets. (Although you think David would have learned his lesson the first time!) But once they find it they are compelled to return to it by forces outside their control. As for Tad and Carrie, they seemingly grew up in that room together, so with the house in turmoil after Tad's return from sea, they may have used it as a refuge from the family and a reminder of calmer and happier times.