I love
Clark for a boy, but
Grace has never done much for me. A lot of people love it, but it sounds too flimsy to me.
I prefer
Clark as a first name, but I don't mind it as a middle name. Either way is fine by me.
Since I'm not a fan of
Grace, I prefer it as a middle name, but it's very overused in the middle name slot these days. It would be a good way to honor your grandmother, though.
Clark and
Grace would work for siblings.
I would honor
Margaret Irene by using either Margo/Margot,
Marguerite,
Irene, or
Irena as either first or middle names.
I'd honor
Mary Elizabeth Mildred by using
Eliza,
Mariel,
Marielle,
Mariana,
Miriam,
Mariam,
Maren, or
Mary. I like
Eliza Miriam,
Eliza Maren,
Eliza Marielle,
Eliza Mary, and
Eliza Mariam.