I've had a normal buffet lunch at the Plaza Gardens, and the character tea party there too.
Mulan is right - at character tea party time, half the restaurant is roped off, so half is for the tea party, half is for normal diners. The buffet service counters are also halved - so the tea party gets the pastries, sweets, desserts and drinks on it, and the normal buffet is served at the other half of the service counter.
I thought the Plaza Gardens buffet was good value for money, for both adults and children. Very good range of salads, starters, and hot mains, including stews, pasta, steamed veg, rice etc. Also some plain chicken etc if your children aren't that adventurous with their food. Excellent range of desserts too.
If you're after dinner at the Plaza Gardens, I'd assume its like the other table service restaurants in the Park - you would have to be there at the latest, one hour before the official park closing time (so for example, if the park closes at 7pm, your latest time for sitting down to dinner at the restaurant would be 6pm). And you would be best to book for that time.
Elaine