As a mobility scooter user, I'm a bit err, confused by this OP. I want to be positive, but it's not really clear whats going on here. It can't really be that you just want to buy one as a toy can it?
Maybe it's nothing, but... is there an actual need for your child to use a mobility scooter?
What would be the benefit be for your child and his disability?
You mention that he could carry more eggs to deliver, which isn't something that sits right with me, unless there's more to it than you've said in that post. If it really is just a case of a child wanting to carry more stuff conveniently, with no link to any disability?, That isn't a reason to use a mobility scooter, as it's not his disability that would be stopping him carrying these eggs?!
He could just as well use a normal scooter, tricycle, bike or even an electric bike. Unless there is something specific that wouldn't allow him to use any of the other ways of getting around?
Do you know that you cannot get him a Class 3 vehicle, due to his age, right?
("The Use of Invalid Carriages on Highways Regulations 1988 prescribes as a condition of use that a Class 3 mobility vehicle user must be aged at least 14 years")
And have you checked out the capabilities of a Class 2 scooter?
They aren't made to go on farm tracks or any uneven paving. The clearance between scooter base and the ground is very low (1-3inches), as they are for people who cannot lift their feet high to get on. They are great in many ways to help someone remain independent but, well designed they are not! And certainly not as an off road kids vehicle.
They struggle to go up and down dropped kerbs in a town or city setting, and they get stuck going up inclines or ramps. They're not even properly waterproof (rain, puddles etc). Did you also know they are also massively heavy with all the issues that would bring.
Even on one of the most rugged class 3 scooters, I struggle to go across well tended ground such as in parks, eg grass, gravel, earthy paths etc. My scooter will get completely stuck on muddy ground, or bumpy ground, and I've needed two strong men to heave it out on a couple of occasions.
If you are thinking of getting it as some kind of toy, then no, it would be very wrong to do this.