In reply to JanPodgornik:
We did a little trip round various parts of NW Scotland a few weeks ago and found lots of sites advertising midge busters. A bit of googling suggests that they are real and that the do actually work, but we didn't experience one in action.
May and June are often OK. June is transitional depending on the year. Normally by mid June it's misery in the valleys, but can be better up high.
Specifically to the OP: they can destroy a holiday, making every activity into a hideous dash between enclosed spaces which must be kept sealed up at all opportunities. It's bad enough if you're staying in a building, but it really steals all of the good stuff from a camping holiday.
I'm not sure where you're travelling from, but it could be a better bet to drive to the Alps or Pyrenees. Plenty of nice places to stay and no midgies. A visit to NW Scotland in late May would be a terrific family trip for next year.