I don't know if it would work with two kids, but I saw a mom who tied a bandanna to her belt and had her little one hold onto it as she made up plates at the buffet. She knew exactly where he was and it left her hands free to carry the filled plates. I guess with two kids, you could put one on each side, or have your daughter hold the son's hand if he'll allow it.
A lot of buffet places won't let kids go up alone, so make sure to keep them with you in line. Your five-year old could probably carry her own plate; just don't fill it up with too many things. You can always make additional trips, which is half the fun of buffets for kids. Some places have lightweight plates or trays so you can manage more than one.
I handled buffets by myself with little ones by first getting drinks, empty plates, silverware, and napkins for all of us and putting it on the table. At the buffet, we'd make up one plate of food to bring to the table. Divvying it up on the clean, empty plates at the table made managing the food easier, plus there was less waste if they didn't like something.