I agree with Parla that the comma after seven o'clock needs to go. It's not actually wrong, I suppose, but it's unnecessary, and there's no reason to have one there. It makes for a slightly awkward break in the sentence.
Whether to take the comma out or leave it in depends on what you want the sentence to mean.
If you take the comma out, it means that on Saturday it was decided that on Sunday they would wake up at 7, but contrary to that decision, they woke up at 10.
If you leave the comma in, it means that on Sunday, instead of waking up at 7 as usual, they woke up at 10 as had been decided on Saturday.
So basically I agree that the comma needs to go, because the second meaning is a bit bizarre.
I can't relate to the concept of deciding when to wake up. You decide when to get up. You could wake up hours before you want to get up, and then lie awake hoping to doze off again.