I usually say "Good morning - on your left" and then " hello" or something as I pass. In fact I usually say good morning, even in the afternoon and evening - that's sort of embarassing!
And I try to remember to say how many of us there are if I'm not alone.
I've learned not to say "on your left" to kids - if they do know left from right, they are so likely to move left! For kids, I just say "Hi there - just stay where you are - you're doing great" or something like that. and the go the safest way around. I think the burden is on me to pass kids safely, not on them to get out of my way. And I guess I extend this to most walkers too, especially coming from behind.
But the cyclists and walkers that bug me are the ones coming towards me 2 or three abreast who do not move out of the way at all. On last week's ride I went off the path twice because cyclists wouldn't go single file.
And I HATE having cyclists blow by with no warning, right or left. So scary.
Keep calm and carry on...