ATV for 8yo and 13yo

Went by the dealer today to inquire about getting an ATV for my 8yo and 13yo for Christmas. Was told that they could only sell me a 50cc for my 8yo and a 90cc for my 13yo. My concern is that I will be wasting my money on getting these smaller sizes and then having to upgrade within a year or two or the kids having no interest in them after a few months because they are too small. Are my concerns legitimate or will these sizes be fine and be able to last at least 3-4 years for each kid?