The great thing I have found about being "Nanny" to my two grandchildren, now aged 6 and 4, is that I could, and still do stuff with them that Mum & Dad do not have time for in their busy lives. I am very lucky to have been very closely involved in their upbringing, as both DS and my lovely DIL work full-time, so I was part-time childminder until the younger one started school this year. They treat my home like their own and can't wait to get here.
The upturn of this is that I seem to have a lot of the fun whilst Mum & Dad have the responsibility. I have spent hours playing and doing daft stuff over the last few years. The other day Oscar, 6, gave me big hug and said "Nanny, I do love you so much." When I asked why that is, he replied "Because you're silly".
Bite your lip, that's always good, nobody will see the bitemarks on your tongue, but look forward to a lot of fun, love and laughter, and get stuck in. They'll thank you for it.