Is your child in some kind of sport/activity where he/she is doing hyper-flexing or overextending type movements?
Could is thus be, activity or sports activity, injuries or charlie-horse type pain?
As you said, your child is getting muscle cramps/pain from "overextending certain muscle groups...."
Or, if your child is in some kind of sports/activity where he/she gets hot and sweats a lot, then it can be something where by nighttime, the muscles are spent and needing a banana to replace potassium etc.
Or, your child can drink a Gatorade or something to replace lost sodium and/or electrolytes etc. My daughter is in Karate, and she gets muscle injuries/over flexed muscles/cramps and all the students bring Gatorade to drink during class as needed. Because, it is very physical and they sweat a lot etc. during class. With Dehydration/loss of electrolytes etc., its effects can and often, will take place, later than same night. And it is hot.
Do you know, that it is not, growing-pains?
I am not an expert, but your child is 8 years old... and you think his/her waking can be from 'separation anxiety?"
And that this is aside from his/her waking at night with muscle cramps?
Also, your question mentions "toe walking" and Cerebral Palsy.
So you are also wondering about these 2 things?
So, Toe walking, Cerebral Palsy, night waking/separation anxiety, and nighttime muscle cramps?