We were advised with both our adoptions that we should meet with the FCs a few weeks after our LOs moved in, so that they could see that they hadn't just disappeared. After that, it was up to us whether to continue contact or not.
In both cases the FCs visited us briefly at our home. The first visit unsettled both the LOs, the elder one especially, but we felt that they would benefit from knowing the people who had first looked after them in the long run, so with the FCs agreement, we had further brief visits at our home and once we felt the LOs were relaxed with that, we also had occasional short visits to their homes.
It was quite nerve wracking to start with, we feared that we would undo all the bonding we had done and with DD we pulled back for a while as she was finding it distressing. In hindsight we took her back to her foster home too soon. It has all come good now though and both the LOs seem to clearly understand that they used to live with the FCs but now they are with us forever.
Now both FC families are like extended families and we attend one anothers children's birthday parties etc. We probably see each family approx four times a year. We think it has been right for our circumstances, both LOs had been with the same FC from birth. Also, we hit it off quite well with both families and they are both fairly local which helped. We felt no pressure from them and both the LOs enjoy visiting with them now and are relaxed about saying goodbye at home time. We knew DD was settled with us when she got tired at a visit to her FC house and she said she wanted to go home and headed for the door.
In answer to the original OP, we told the children that the FCs would be visiting to see how they were on the day that they were coming. DS was only approx 1 year old at the time and was distressed when they first arrived but was relaxed when they went. DD was 2 and was very pleased to see her FCs but very distressed when they left and talked about them a lot. We found after that, for a while, she would talk about them more and more, like her anxiety was building, but that a visit cleared it for her for a while- think she was reassured when she saw them.