Most places give you one option or the other. Say you want no payments for 6 months then you pay cash at the end of the 6 months. If you plan to use a ktm card you probably pay interest after 45 days. Best thing to do is go to your dealer and they can explain all your options.
If your going make payments to get the best deal would be to get a credit card with 0% interest for 6 months, before that 6 months is close to being over, apply for another card with 0% interest and transfer it. Then before the 2nd card has used up its free interest you can call the 1st card company and ask them to give you free interest again if you transfer back, some will do this and some won't. Its a lot more risky to juggle cards then to pick a fixed interest rate but it can save you money. Quads/motorcycles tend to have pretty high interest rates. Normally those real good rates they put out are only available to people with excellent credit. If you mom has enough in the bank to cover it, let her buy it and make her payments

You may also have to pay insurance if you get a ATV loan. Just some more stuff to consider.
-Voodoo