The Shelah HaKadosh strongly forbids bringing small children into the Beis Knesses until they are old enough
to learn how to honor the Beis HaKnesses properly (Not crawling under seats and bawling)
The Mishna Berura nad the Magen Avraham quotes the Shelah HaKadosh in 98:1, thus both holding his opinion.
The Kitzur Shulchan Oruch by R’ Shlomo Gantzfried says that men, women, whether married or not, old or
young, but the only exception is bringing a very small child, because it will disturb others (141:7)
The Rema in HaMapah 98:1 says ‘It is forbidden to kiss one’s SMALL SONS INSIDE THE SHUL, in order to show
that there is no love like the love of Hash-m’. Small sons: Either 1) The Rema permits small children in a shul
or 2) The children are SMALL compared to their adult father