Tell them whenever you want. They're not children, they'll understand that things can go wrong early on. Bear in mind that once they know they will get as invested in the thought of being gps as you are in being a parent.
The first time we told them at about 7/8 weeks and they were absolutely over the moon, overjoyed, elated. Having to tell them three days later that there was no heartbeat on an early scan was the hardest thing.
We then went on to have DD a year later and I honestly can't remember when we told them, possibly after we'd seen a heartbeat in an early scan, or maybe after the 12 week one - I was far too paranoid about it going wrong again to tell many people early.
This time, just before Christmas, we only told them after we'd had an early scan and knew the embryo wasn't developing. My mum was lovely throughout, the whole thing dragged out over a few weeks, but I'm glad I hadn't got their hopes up first, it was much easier.
Ultimately we don't see them every week, although we talk almost daily, and the only real practical support I actually wanted/needed was from my dp and a couple of friends.