"Meinem Kinde" (To My Child) by Strauss, Op.37 No.2 is a good one, and suitable for a soprano I think.
"Les Berceaux" (The Cradles) by Faure is about how Mothers are left rocking the cradles while the men ###### off.
"To the children" and "Child, you are lovely like a flower" by Rachmaninov are beautiful but these are more mezzo-y (and Russian - difficult if you're not used to singing in this language.) Oooh, and "The Soldier's Wife" also by Rachmaninov is a beautiful one, but obviously more wife than mother.
"Smutna Rzeka" (Sad River) by Chopin is hauntingly beautiful and has something about a Mother in it but its in Polish, so again maybe a bit difficult.
An option from the musicals repertoire is the Marilyn Monroe song from Blood Brothers. Talks about Motherhood and other aspects of being a woman. More specifically about Motherhood are "Easy Terms" and "My Child" also from Blood Brothers, but I think these have parts for two singers, although there might be some way you could cut it at an appropriate point or summat.
Hope that helps!
Wow, sorry, I didn't realise that was a rude word but the ABRSM seems to have blanked it out. Sorry, it honestly wasn't that rude. I'm kind of embarrased!