Y.K.
I have a Schwinn bike trailer for two children. It comes with a built-in stroller wheel so you can easily use it as a stroller. It comes with the handle bar too. I use it as a stroller and also with my bike. It works with well with my 2 and 4 years old, who are 30 and 40 pounds. I had a Graco double stroller when they were little. Now the kids get more room in this bike trailer/stroller. What's more is it's easy to push too.
I found it on clearance in my local retail store ShopKo, which is like Target. It probably costs between $120 and $200 if you can find it at Target or some place like Target.