When it comes to television, studies have shown some negative correlation. But they disappear when you adjust for socioeconomic status and parental education: http://www.cnn.com/2009/HEALTH/03/03/babies.watch.TV.
What the data really indicates is that poor people and single parents let their kids watch more TV, and since IQ is highly heritable, TV watching ends up being linked to lower cognitive performance. But it doesn't seem to be a causal relationship, or at least the data doesn't clearly support that conclusion.
Obviously its important to engage with kids, and parking them in front of an iPad could get in the way of that. But at the end of the day, there are no prizes for making parenting harder than it already is.