As others have pointed out... Some would go stir crazy on Ko Samui after 10 days
My first trip was 2 nights solo, second was 3 nights with BF... He has officially banned us from going back there!
Why not Spend the first 5 days with your friend and her son on Samui, and then fly over to Phuket for the next 5?

I note that the Akwa Guesthouse on Chaweng/Samui has closed down.... good riddance!
We booked there after advice on this forum and ended up moving out and forfeiting our prepaid rooms 6-7 hours after checking in!