CONGRATS Pro-Z & welcome to the club. Now as the offical "GRAMPA" rule book reads...
1) you must never tell them "NO"
2) you MAY spoil them rotten....
3) when they DO get on your nerves - you give them to their parents and say "they're getting tired - they need to go home now"
(works every time)
We'll talk more at Eddyville - my friend