Waking Up Early

If the kids don't have a bed time, what do you do about days when you have to get up early to do something?

Josh (8y) typically goes to sleep anytime between 10pm-midnight and will wake up anytime between 8:30am-11am.

Me- Hey Josh, Grandma wants to know if you would like to go to her land with her tomorrow.
...
Josh- Yeah! It's been forever since I've been there.

M- Alright, she will be here at 10am to pick you up. You'll need to be sure you're awake in time to get dressed and eat something before you go.

J- Okay, I'll set my alarm for 9:45am.

M- Is 15 minutes enough time for you to eat and get ready?

J- Hmmm, probably not. What time do you get up?

M- Usually around 8:30am.

J- I'll set my alarm for 8:50 I think.

Which he did, and he got up with no problem. It was important to him so he was motivated to do it. If we were getting up for something important to me we'd likely need to negotiate a time we could both agree on, or come to some other such agreement to benefit Josh as much as me.