We can be confident that a child who professes faith in a Reformed church has true faith because of the Reformed tradition’s emphasis on the right administration of baptism. This might sound a bit circular or truistic or redundant (I don’t know what the right adjective here is). But it is true.
I have heard many stories about a popular church on island that does not carefully administer baptism. Said church has baptism services at the beach where people have signed up last minute to be baptized, sometimes because of being peer pressured by friends to sign up, many baptism candidates have not been interviewed before the day of the baptism (I have no idea what the conversation with the candidates are like on that day), and once I even heard a candidate came drunk to the service, was refused by one pastor, but allowed by another pastor on the same day.
By God’s grace, we try to be very clear with the gospel each Sunday, and very clear with a call to repentance and faith in response to the gospel. And with repentance, we actually try to find actual sins the baptism candidates can actually repent from. If there is an unwillingness to repent of anything, we would delay baptism. In my mind, that is pretty careful, and at the same time, pretty simple.
We also make baptism entrance into church membership. I repeat: it is entrance into church membership. It is not pre-requisite to membership; it is membership. The church and new member make a covenant with each other in their baptism. They are a member the moment they come up out of water.
Therefore, we do not baptize anyone who is not willing to be a committed member of the church. That goes for kids too. They must open themselves up to the accountability of the church, they must be willing to confront others about sin (or at least talk to their parents about another member’s sin), they must be willing to submit to the elders, and they must be willing to serve. In other words, we only administer baptism to those who are ready to become followers of Christ. So if there is a basic understanding of what it means to be a part of a local church, and they still want to be baptized, I do not really know how much more careful you can be (except for too careful).