While there are many poems written about mothers and even about fathers, it's unusual to find a poem about grandfathers. Poems about grandmothers are much easier to find. Certainly one can just adapt a poem and insert "grandfather" or "grandpa" in there, but oftentimes the rest of the words don't match anymore.
One good kiddie poem written about grandpas is by Joanna Fuchs. She writes:
. Happy Valentines Day to My Grandpa
You are the greatest grandpa;
We have lots of fun.
If there were a grandpa contest,
You surely would have won!
Thanks for being such a good grandpa!
I love you.
Her poem is a sweet one in its innocence and doesn't try to overly describe what her grandpa is like or his role in her life, which makes it simple and sweet for children everywhere. Other poems try to define what it means to be a grandfather, such as this by Bobbie R. Bringold:
Definition Of Grandpa
Having an answer for every question
Understanding more than I ever knew,
Glad to help relieve any form of stress,
Happy no matter what there was to do.
Placing fake bets on silly horse races,
And always with a smile,
Trying so hard to make everyone happy,
Regardless of an extra mile.
Intentions always on the good side,
Careful to always say the right thing,
Knowing more than we'll ever know, but
Maybe he's above us with a pair of wings.
Unlimited love to us he gave,
Living life one day at a time,
Having difficulties to overcome
Oh, if life were as easy as rhyme.
Leaving us wasn't his choice at all
Loving us forever was,
And in our hearts he remains,
Never again to leave us.
Dearest Grandpa, you're always missed and are forever loved.
Such bittersweet poems attempt to speak to everyone with an experience or memory of a grandfather, while still discussing it with the childlike nature with which most actually experience their grandpas.
Another simple one is:
You're More To Me
You're more to me than my grandfather
You are also my best friend
You're always there for me
For an ear or a shoulder to lend
You're the first to hear of my joys
It's you I go to if I've had a bad day
I love you more than I have words
To express myself or say
- Anonymous
http://www.poemsource.com/kid-valentine-poems.html or http://festivals.iloveindia.com/fathers-day/poems/poems-for-grandpa.html