You might want to think about a day-by-day rental if the savings are so small, or possibly just a single. Our two youngest are nearly 6 and 4 1/2, and we felt the same way about their stamina. But we seldom pushed them and never needed to push both at the same time. A bigger issue for us is keeping them content while they wait in long lines (I don't think Disney rents anything for that).
A couple of thoughts ....
Regardless of its listing in AllEarsNet, there really is no such thing as "length of stay" rental, or rather, "length of stay" can be a misleading term.
The CMs at the stroller rental shops at the 4 theme parks don't know what your length of stay is, and they don't care. They just want to know how many total days you want a stroller for. So in other words, if you are staying for 11 days but will only need a stroller in the parks for 7 of those days, you don't pay for 11 days, you pay for the number of days you want. That's why I think "length of stay" is a misnomer, because some people might complain about thinking they have to pay for every day they are staying even though they won't be in parks each of those days.
Anyway, to comment on what ammo said ....
Stroller rental fees are completely refundable. You can get your money back in full for each day's worth of stroller rental you don't use. When you do a multi-day rental, you get a rental pass for each day you paid for. When you initially get a stroller each day, one rental pass is stamped with that day's date on it. You can get a refund for any rental passes you have that you didn't use (the ones you didn't use won't be stamped); you will also need your receipt for this transaction, and the receipt will be stapled to the strip of rental passes when you first get them.
With this knowledge, you'd know it would be a waste of money to go day-by-day to decide if you'll
rent a stroller or not. You can pay in advance for multiple days, take advantage of the multi-day discount, and then get a refund for any unused days. It will also save you a lot of time in line each day. The "pay" line can get rather long sometimes, but if you already have an unused rental pass you totally by-pass the "pay" line and go directly to the always short or non-existent "pick-up" line.
The stroller rental shops inside the 4 theme parks accept credit cards and room charges as well as cash.