Q: Why should Catholic babies be baptized?
A: Children don’t have to understand the significance of the sacrament of Baptism to benefit from it. This sacrament’s great value lies in what it does to the soul by bestowing on it sanctifying grace which makes us co-heirs of heaven with Christ.Through Baptism the life of Jesus becomes our own. This means that we are raised to the level of the divine, which we could never achieve on our own.

We don’t wait for children to understand the value of vitamins before we give them to them. Why would we ever want to wait to give them the greatest gift possible—life in Christ?

- Fr. Vincent Serpa, O.P. of Catholic.com

Other less important things babies don't understand yet we grant to them: first birthday party, branded clothes, sometimes even a watch.

There was one baptism rite I attended as a substitute (proxy) godfather. The presiding priest had this message:

"Does this baby know Jesus? Not yet. Does Jesus know this baby? Of course. Do the parents know Jesus? Yes.
Does this baby love Jesus? Probably not yet. Does Jesus love this baby? Of course. Do the parents love Jesus and the baby? Yes.

Then whoever loves the most makes the first move. Very applicable not just in baptism.

- EO