I know there are discussions in the paedobaptist Reformed world about presumptive regeneration, but most paedobaptists I personally know would not put it in those terms. "Presume" is too strong a word. It suggests a certain gullibility that I don't feel I have toward church kids.
I treat the children of believers as young disciples. They are part of the church. They are part of Christian families. They are learning to follow Christ and are growing in faith.
The way I use the word "presume," this is different from presuming they are regenerate. I don't presume anyone is regenerate. That's God's call, and I don't want to get presumptive. But I do treat church kids as fellow disciples.
When I say I treat them as disciples and part of the church, I mean that I speak to them as I would anyone in the pews. I don't constantly question their salvation, which would destroy their confidence in Christ. Rather, I usually speak to them as if they have all the benefits of salvation, and occasionally I remind them that one does have to believe to be saved—just as you probably would if preaching to the whole congregation. It also means I speak to them with a measure of faith—not faith in them but faith in God. I have faith that God works in families and blesses those who are part of his church. Again, it is not a gullible, presumptive faith that's blind to the fact that God may place people in the church who are not actually regenerate.
So I'm not making any presumptions you wouldn't make about any other baptized person in your congregation. I'm guessing you treat those people as fellow disciples who are learning to love Jesus. In your interactions, you generally treat them as you would a treat person you knew was saved (if you could know such a thing), but you remain aware that despite their church membership and baptism they might not actually be regenerate. You don't get too presumptive, in a gullible way. You pray that they truly know Christ.
I interact with Baptists who sometimes question how one could baptize a person before they are saved. I counter by asking how they could baptize anyone if that's the standard to be proven. The Presbyterians I've been around baptize not on a presumption of regeneration, but upon a credible profession of faith (either from the person being baptized or, in the case of a child, a parent) and with faith in God to work in families and in his church.
The church is made up of baptized people, is it not? And baptism precedes discipleship, does it not? That's why, in my view, church kids should be baptized. It is not because we've been able to judge their hearts or are presuming something about their hearts that is God's alone to say.
Anyway, that's how this ordinary paedobaptist phrases things. There are folks on the board who can parse the whole "presumptive regeneration" phrase more expertly than I can.