I think there are a few 'studio plus' rooms at BWV that take 5. The 5th 'bed' is just a small bench with a foam pad on it ,though.
There are 1 BR villas at AKV that 'officially' take 5, but MS has been letting people book 5 in 1 BRs at the other resorts as well.
OP, if one of the kids is under 3, you'll be fine. The occupancy rules for a studio are 4 guests, plus 1 under 3.
If both of the kids are over three, you may run into problems, and not just with Magical Express. The DME vouchers are only sent for those who are on the reservation. Kids younger than 3 don't need a voucher to board the bus. Now,you may get lucky and get CMs who don't bother to check all the vouchers, but be sure you have a backup transportation plan in place if you're going to try that.
Only those on the reservation get resort IDs, which are needed for access to EMH. So, if you're leaving one kid off the reservation, you won't be able to take advantage of EMH.
Some resort pools do random checks of resort IDs as well.