I eat at Milestones quite often as it seems to be the default location for birthday parties these days. But for the amount you spend, you could easily find nicer places to eat at. I would generally suggest staying away from chain restaurants when it comes to big groups (ex: birthday parties) because they're always busy, especially on weekends, and they very often don't have a choice but to cut corners in the preparation of their dishes in order to accommodate for larger parties.
With all that said, if you must eat at Milestones or near the Eaton Centre, I would recommend this location. The food at Milestones is really nothing special, and neither is their coffee. What I do like are their bellinis and their cheesecake is also quite decent, but they probably order that from some other place anyway. This place however, has me sold on their atmosphere. Their patio is lovely with an overlooking view of the busy Yonge & Dundas intersection and their bar area is also really nice and spacious. I also really love all that natural light coming through the oversized windows.