stay part doesn't add much since the room rate for two adults and two kids that age is the same as just two adults. So the savings is for the park tickets for the kids.
It varies based on the level of resort you are staying at and the length of time you are staying. Typically 40% discounts are offered on only deluxe hotels and often not all of them.
Here are some examples for three different per room price levels - $300/$263/$167/$120.
4 Days and 4 Nights - 2 Kids Tickets $420
$300 with 40% discount and kids tickets - $1,140
$300 rack rate no kids tickets - $1,200
Savings $80
$263 with 40% discount and kids tickets - $1,051
$263 rack rate no kids tickets - $1,052
Savings -$1
$167 with 40% discount and kids tickets - $668
$167 rack rate no kids tickets - $821
Savings -$153
$120 with 40% discount and kids tickets - $480
$120 rack rate no kids tickets - $708
Savings -$228
Since ticket prices go up only about $5/day above four days the savings are for kids stay free.
6 Days and 6 Nights - 2 Kids Tickets $440
$300 with 40% discount and kids tickets - $1,520
$300 rack rate no kids tickets - $1,800
Savings $280
$263 with 40% discount and kids tickets - $1,387
$263 rack rate no kids tickets - $1,052
Savings $191
$167 with 40% discount and kids tickets - $1,041
$167 rack rate no kids tickets - $1,002
Savings -$39
$120 with 40% discount and kids tickets - $872
$120 rack rate no kids tickets - $720
Savings -$280
The summary of all of that is that at four days if the resort cost is about $263/night or higher the kids stay and play makes sense. Sometimes you can get AKL or WL for those rates but usually the 40% discount applies to the more expensive deluxe resorts. For six nights the breakeven nightly resort cost is $183 - or a moderate resort - but I've not seen a 40% discount for them.