Q. I attended a bris where the sandek was sitting on one of the tables of the shul, since they couldn’t find anything better for this particular sandek. Is that allowed? Are you permitted to sit on a table, that is like a mizbeach, specially if its from a shul and used for seforim?

A. Remoh (O.H. 167: 5) mentions that it is tradition to place salt on the table, since it is compared to the mizbeach. Therefore, Magen Avraham (ibid. 13) maintains that one should refrain from killing an insect on the table; while Kaf Hachaim (ibid. 141) prohibits out of respect, letting animals feed directly from leftovers that remain on it.
Sefer Chassidim (920) quotes a story of a child hurting himself while standing on a table also used for placing seforim and learning. Therefore, Yesod Ometz (64) maintains that one should not sit or stand a child on a shtender where seforim are usually placed. (on Yosef Ometz 2: p. 278, he seems to contradict himself). Yeladim Kehalacha (p. 48) writes that some abstain from letting a child seat on a table even when no seforim are on it. Maadanei Asher P. 59) quotes the widely recognized notion that seating on a table is improper.
Nevertheless, Horav Shlomo Miller’s Shlit’a opinion is that if no seforim or tefilin are now on the table, since it is also used for eating and reclining on it, it could be used for sitting the sandek when performing a bris milah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a