LA-
There are not many only children familys in the trad movement because most do not believe in cessation of generation; in limiting 'how many' children to have. They may space their children out for financial reasons, but we are never permitted to cease having children once we feel, "Two [or three or four] is enough for me!"
The Catechism teaches us that the primary end of marriage is the production of children. Love and companionship is secondary.The second house, the boat, the vacations- all come far after. Our modern, secular, world teaches us the EXACT opposite if you think about it.
Would a single child family be looked down upon- I would hope not. Most trads understand that many come to Tradition from an upside down NO Rite, and expect many to have practiced some sort of contraception- be it vasectomy, Onanism (the man spilling his seed other than where it was intended, at the end of the marital act), condom use, or (May our Lord have mercy on us) the use of the pill- which is NOT contraceptive, but abortive.
Many times, if the married couple includes a man who has undergone a vasectomy, the priest will most likely encourage him to have it reversed; even if the doctor states it would have no effect.