Grammatically, I concur with HalvarF's answer.

To provide a different perspective from a native speaker living in Germany about what would be usual to say, I'd argue even "Ich habe dich lieb." sounds way over the top. It would be appropriate in a romantic context again, or maybe among groups of (usually) younger girls.

The question is how

to express a mother's love for a child

in German. I think there is a very simple answer: You don't, at least not in an explicit declaration. It sounds extremely unnatural every time I hear it in a dubbed American TV show ("Why do they keep saying that?! They're parents and children, it's clear that they love each other!"), and as far as I can tell, declarations of the sort "I love/like you" to one's parents or children are something that is simply not done in German.

What is indeed somewhat usual is rather some sort of "praise" pointing out the special meaning of the addressee to the speaker:

Du bist die beste Mama der Welt!

(You are world's best mom!)

Du bist meine absolute Lieblingstochter!

(You are my absolutely favourite daughter.) (Use if you have only one daughter; this has the added benefit of being slightly humorous. In case of several children, you might want to pick a less exclusive wording such as "... elder/younger daughter!")

Depending on the situation/content of the preceding conversation, some more context-specific statements are also possible:

Ich freue mich auf euch!

(I'm looking forward to seeing you!) - if there's an upcoming encounter with the addressee.

Du warst mir eine riesige Hilfe!/Du hast mich gerettet!

(You've been a huge help for me!/You saved me!) - in case the addressee provided some help to you.

Hoffentlich können wir uns bald wiedersehen.

(I hope we will see each other again soon.) - to express you're missing the other person(s). Somehow, an explicit "Ich vermisse dich/euch." (I miss you.) sounds quite blunt to me.

P.S.: I'm on very good terms with my parents, and have always been.