I propose that for some of these minor days we follow the example of the valiant Europeans trying to fight them off.
You can treat your grandparents, your administrative assistant or your boss any time. Setting up a day that's not that widely known might just set the potential recipients up for disappointment.
As for "children's day," when my children were really young, a casual friend invited me to a gathering in a park as "a day to honor our children." I was like, "Oh, for god's sake, don't we do enough of that all the time already?" I didn't go.
The statistics might surprise you. Both mothers and fathers, including those working full time and otherwise, spend more time with their children now than they did in Beaver Cleaver's day. Mothers spend more than fathers, of course. But both spend more time than earlier generations did, even when the phrase "stay-at-home mom" would have seemed weirdly redundant.