If it's an old-fashioned canvas tent that doesn't zip shut all the way around the doors, it's essentially impossible to keep all the critters out.

If it's a more modern tent of nylon or other non-canvas fabric, DO NOT apply insect spray to in or on the tent. It can dissolve the waterproofing and weaken the fabric.

You might be able to find a short broom for getting the bugs out of the high reaches of the tent. We have one that looks like a whiskbroom with a three-foot handle.

And now for the technicalities--daddy long legs, also known as harvestmen, are arachnids rather than insects. There is an insect known as the crane fly (they look like gigantic mosquitoes), but it is no relation to the daddy long legs.