The main issues I see are the size of wheels, the tyre compound and the gear-ratio. The tyres look very hard and plastic-y, so they probably won't offer much grip. May be worth running a layer of bike tyre around them to improve grip. The wheels are also very close together, which will make turning difficult. The gear ratio from the motor doesn't look that large for the size of wheel so torque may also be an issue.

Obviously changing the gear ratio would be extensive work so maybe try screwing some bike tyre onto the wheels first to see if that improves things. Halfords often have old tyres in the back from bike services that can be obtained for free.