Dear Grandma,

Hi there! First, this poem is very sweet and it's lovely to hear you have such a great relationship with your grandma!

I do have a couple of pointers though, just where the rhythm is wobbly in places:

Dear Grandma,

Remember when we used to play
with the dollies on my bed? <<< Try to keep a set rhythm, you've got two lines which have eight syllables, one with nine and one with seven but if you make them all eight then you'll see a crazy improvement in the flow. For example, this one could be 'with the barbie dolls on my bed?'
You helped me dress them and you listened <<< And this could be, 'When we dressed them and you listened'
to everything I said they’d said.

Remember when we used to pretend <<< The flow is a little awkward here. Maybe 'Remember when we played pretend', then change the 'that' and the next line to 'and'.
that I was your masseuse?
Or when grandpa would tickle me ‘til I turned red,
and you’d make him turn me loose.

We used to watch old musicals,
you’d give me sherbert in a cup.
You used to call me Guppy, <<< Could use an extra syllable, maybe throw 'how' in there, at the beginning.
that is, until I grew up. <<< This line feels really awkward. I'd suggest something like 'but eventually I grew up'

We’d count those tiny velvet bears
and put them in their house.
Then you’d read me stories like Rainbow Fish, <<< Too long! Perhaps, 'Then you'd read to me from Rainbow Fish'
or The Lion and The Mouse.

You were always there with a comforting word <<< Maybe, 'There was always a comforting word'
whenever I was sad,
you’d hold me close and dry my eyes
singing the song you always had. <<< This is another awkward line. It feels too forced, try to avoid that. Perhaps, 'and sing the song you had'

You’d read to me or sing to me
to get me to rest. <<< Break in the flow here. Try: 'when you needed me to rest'
But in a flash you’d be by my side
if I ever was distressed.

Although I have grown up much since then,
That is undoubtedly true.
Remember you’re my Grandma
and I’ll always cherish you.

Very lovely ending! I've not much to criticise about this as it isn't one of those poems you want to edit to death. Just tidy up the flow and it will be perfect just the way it is. Hope this helps,

Heather xxx

Aww! My grandma used to read me Rainbow Fish.