I really play it by ear day-to-day. We try to get outside daily, at least for a little bit. Usually after naptime, or in the mid-morning. So we do avoid the hottest, most sunburn-y time of day.
The yard surrounds our house, so at any given time we can usually find some shade. But I make sure we have lots of water, everyone wears a hat, and I watch the kids pretty close to see that they don't get overheated. Sunstroke/heatstroke SUCKS!!!
@CanadianJule: I feel for ya, that doesn't sound very fun. Looking for a tent is a great idea. And at least you can maybe use the rain barrel to fill up a little splash kiddie pool for the hottest days?