So this may be an old trick for you more skilled Mommies, but this is something that just "dawned" on me the other day!
My daughter loves aprons - when she's helping me cook, playing dress-up, squishing Play-Dough - you name it, she's running around in an apron. But since she's still so little, most aprons are too big for her! I've tried tying a knot in the neck strap, but she doesn't like that, says it's not comfortable. *sigh* Thus, this new way!
Pull the apron up in the front, allowing the neck strap to sag in the back.
Pull the side ties through the neck strap from behind.
Tie a bow!
And voila! Your apron now fits!