It all depends on a few factors, imo:
To what extent are iPads used? Too much certainly isn't good.
What is it used for, and how is it used? There are certain sites/apps, ... that are not meant for children and should be restricted.
Is somebody supervising, or are the kids left alone with the device, and nobody controls them? This is the most important one, I think. There are enough parents who use tablets to keep their children quiet, but don't know what they're playing or looking at.
A computer/tablet/smartphone can't replace social interaction. I've already seen computer savvy children, (not always, but often) ignored by their parents, who are outsiders, because they don't want to understand that there are certain things that must not be said or done when you talk or play with others. (That's extreme, but really happening.)
I'm a primary school teacher. I know from most of my pupils, that they are playing games on their tablets (they have their own), but only one of them is actually reading ebooks.