are you feeding them both a AM snack and Lunch or us an AM snack?
are you going to be doing crafts or be outside all morning?

Snack + Lunch = $6.00/day
Snack = $2.00/day
Lunch = $3.00/day
Crafts = $2.00/day

So if Snack,Lunch and crafts you will be spending around $8.00/day/child
$20 - $8 = $12 left as income

Snack and crafts = $4.00/day
$20 - $4 = $16 left as income

$20 isn't too bad. But I wouldn't go any lower myself