Jack and Luke live down the street from us; we go to church together. Jack is a little bit older than Elena, and Luke is in between Elena and Roman. They have twin baby brothers who were born about six months ago, just before we moved here. Because the twins were premature, they weren't allowed to leave the house much until recently and as a consequence Jack and Luke don't really get out as much as they, or their mom, would have liked. Once Jenny realized that, she started to drop by their house any time she took Elena and Roman to the park, and offered to watch Jack and Luke whenever it would be helpful. This arrangement has worked out really well for everyone.
Going out to play more is certainly good for our kids; I imagine it's the same for Jack and Luke. The greatest benefit I've noticed for Roman and Elena is just how nice it is to have friends that live nearby. They can just knock on the door and ask if Jack and Luke want to play, without having made any prior arrangements. We have to drive by their house on the way to and from ours, and Roman always perks up, points at it and says "Jack, Jack, Jack!" It's obvious that he likes and admires the older boys. Elena, who is naturally sociable and friendly and frequently needs to give goodbye hugs at the playground to children she only met thirty minutes before, but who have become her dear friends in that short interval, enjoys this longer-term friendship just as well.
There is one possible downside: the toys. For one thing, Jack and Luke have more boy-focused toys than we do. Roman loves balls--basketballs, footballs, soccer balls, and more--but until recently when Jenny scored some on freecycle, we didn't have many at home. Roman would easily become upset on the playground when other kids had balls and he was the one left with nothing. It was sometimes hard to remove him from Jack and Luke's house peacefully while leaving their balls behind, but now Roman does have some of his own to carry around whenever he wants.
I don't mean to imply that arranged play dates aren't good. Elena's and Roman's friends Saoirse and Maeve live about twenty minutes away from us, but they enjoy each others' company. Jenny and their mother Lauren have a longstanding weekly appointment for the children to have a play date. They alternate weeks, and while one of them takes care of all four kids, which can be exhausting, the other gets a whole free morning to do whatever she wants, whether that's running errands, relaxing, or anything in between. It's a good deal for everyone involved.