As a former Monorail Pilot, I can tell you that only FOUR guests are allowed in the front. Big, small, young, old, doesn't matter. Once they've been born, they count. Monorail Pilots are *VERY* strict about this rule, and not even Management will budge on this. It keeps everything fair for everyone, and minimizes any disputes.
At one time, more than 4 were allowed. However, for safety reasons, the limitation was put onto the front cab many years ago. (I can't go into details as to the reasoning.)
The rule is strictly enforced because we have SO many problems and situations with the front cab. It's sad, but Disney Security and even Orange County Deputies often have to be called to resolve front cab situations. In order to minimize any problems, the rules have been created, and are strictly enforced. This way, someone can't go "Well, THEY had 5, why can't we?" or "We did it last time!" Every ride is as fair as possible.
Strollers are allowed in the front, so long as they can be folded, and are somewhat compact. Certain larger strollers simply will not fit in the front cab. Those extra large strollers may be placed in the car behind the front cab.
On a similar note, guests using wheelchairs or other mobility devices are also more than welcome to ride in the front, as long as they can transfer from the wheelchair. The wheelchair would then have to be placed in one of the cars.