That will work and provide power to the wagon however it is not recommended for many reasons.
It is not recommend using a car starter battery in a series wiring arrangement with sealed lead acid batteries because they are bound to be different sizes so the larger battery will deplete faster than the smaller batteries do which could cause damage to the smaller batteries from being over-discharged. Also mixing different brands and sizes of lead acid batteries together into a single battery pack is not recommended because the different chemistries and electrical ratings of the batteries may not be compatible with each other which could cause the batteries to wear out faster than normal.
Another thing to consider is that car starter batteries are not designed for deep cycle use such as they will encounter when used on an electric wagon. They are designed for short bursts of high power to start an engine, and then to be immediately recharged. Starter batteries have thick lead plates which can output high amounts of current for a short period of time, as opposed to deep cycle batteries which have thin lead plates which can output low amounts of current for a prolonged period of time. If a starter battery is used for deep cycle conditions then it will not perform very well and will not last very long.
None the less if you have a couple of used starter batteries laying around that are not being used then there is no harm in making a 24 Volt battery pack out of them and seeing how long they will power the wagon for.