Here are a few ideas.
Tell them what you admire most about them.
Write about their interests and the things they love to do - things that fill up their days now, but in 10 years time might feel a bit like a hazy memory.
Tell them how it felt when you got to hold them in your arms for the first time - what you'd been dreaming of and hoping for while you were pregnant, and how it has felt watching those dreams unfold over the years since they were born.
Tell them how similar they are to yourself - or how different.
Tell them what you love doing together. Tell them how much their bedtime cuddles mean, or how it makes you feel when they wrap their arms around you and say they love you.
Tell them what they've taught you.
Tell them how much more wonderful your life is from having them in it.
Out of all the people in the world, what made you choose them to venture on this parenting journey with? How has being a parent changed your relationship? What do you love most about them? What most melts your heart about watching them with your children? (Double bonus points if you can get the other parent to write a letter to you in here too!)
What does it mean to you, being part of this little family? What do you love doing together? If you had a perfect day to spend together, what would it involve? If you had a family soundtrack, what song would it be? And most importantly, what do you MOST want to remember about your family at this stage in your lives?