In all honesty I don't think a poem would mean as much as your words, the cards I have that mean the most are the ones from people who just simply say how sorry they are or shared a memory of my son or from when I was pregnant with my daughter.
I usually write a poem on birthdays or angelversaries for friends who have lost children.
there is a great thread on a memorial site here with lots and lots of poems on it, there are religious and non religious ones so if you do want a poem you should be able to find one that is suitable on there.
Time spent with you Was so very precious Even if only for a very short time I hold a special memory Close inside this heart of mine To me you were very special Much more than words can say I still love you now little angel And I'll remember you every day.
i think I'd be totally honest and write in the card that you have no idea what to say at this horrible sad time but that you are thinking of her a lot. say you know how loved he was and how lucky to have her as his mum and finish off by saying that if she wants/needs anything she only has to ask.