Other option is consider a wagon for the summer meaning the extra child could ride at least part of the way and would also give a place to dump the bag of sand toys or snacks you take to the park. Then by Sept your own child would be older and able to walk more and you could make more use of the stroller.

The downside to getting an older child is that often mom is pregnant again and not saying, or thinking about it or the child got kicked out of their daycare for some reason and are a problem you don't want. A newbie that you mold from the beginning has so many benefits.