I've had kids come in during art and snack/lunch times. I always give them the choice unless it's the very end and I'm cleaning up.

I pretty much always know what time this group of kids (in the past I've had parents drop off whenever) come so I just plan around that if need be. I'm not overly set in my times as long as it's within 30 mins or so of the regular time.

I agree with lee bee....hungry kids are not fun and I'd rather give them snack than deal with a meltdown when I'm getting lunch ready lol.