That's the plan. William's eldest son will be a prince. The younger children will be given, as I understand it, the styles and titles of the offspring of noble dukes -- "Lady" and "Lord". That would be the case until Prince Charles became King, whereupon William's children would ALL become princes and princesses.
What isn't clear is what might happen if the first child is a girl. It's very likely that the UK will adopt absolute primogeniture, whereby the eldest child, male or female, will become the heir to the throne after William. If William's first child is a girl, and if Charles has not yet succeeded to the throne, then presumably, she will be called "Lady Whoever," but since there's a good chance that she might be her father's immediate heir someday, the Queen might issue Letters Patent to make her a princess instead.
The Queen can, of course, make changes to her grandfather's decrees regarding title protocol. That's part of what being the sovereign is all about.