Yes you get a +1 bonus for being mounted and attacking a medium-size creature or smaller. (see PHB pg. 138, Combat while Mounted).
Note, if you are using a lance when charging it makes sense to hit one opponent, but not more than one. If you are using a melee weapon, then you should be able to Cleave with the charge bonus.
I would modify your feat as follows:
Mounted Cleave [General]
You can use the speed of your mount to close in on additional opponents.
Prequisites: Str 13+, BAB 8+, Cleave, Mounted Combat, Power Attack, Ride by Attack.
Benefit: When using the Ride-By feat, if you drop (typically by dropping it to below 0 hit points, killing it, etc) the target of your Ride-By attack you gain an additional attack against another target along the line of your charge. You must be using a non-lance melee weapon to take advatage of this feat. You do provoke an attack of opportunity from your second target.
The extra attack is with the same weapon and at the same bonus as the attack that dropped the first creature. You can use this ability once per round. Note: All movement of the charge must be in a straight line as per the normal charge rules.
In my mind this simulates the calvary charging through a bunch of mooks and hacking them with his sword as he rides past. I don't think that you need the 10 foot movement restriction. Also, you could create a similar feat based upon Spring Attack and Fly-By attack.