It’s all relative I suppose. I’ve gone for walks in the park with people who don’t get out much and considered it a hike, and I’ve gone for hikes that felt like walks, just because I was in good shape at the time.
Hiking involves hills and the outdoors. Hiking tends always to work up a sweat. Hiking also usually leads to some sort of aesthetic or internal reward at the end. Though a vigorous walk can work up a sweat and could lead to a pretty view and make one feel better about themselves on the inside—here I go again back and forth.
Splitting hairs really but hiking always seemed like a somewhat more considerable investment of time and physical effort.
How old are your girls? Are they wee ones? They might consider any sort of walk in the woods a hike, and all the better for it, it might pique their interest in backpacking or ski touring later on—both of which are very rewarding.