What they need to start knowing about is the relationship between numbers - e.g which numbers are bigger or smaller than other numbers etc. when they have enough of one thing to match with another etc. Simply recognising numbers up to 100 is no indication at all of any understanding about number.
Reception classes focus on this understanding of the relationships by using numbers up to 10 (or 20). Once that is secure, they help children to learn to apply their knowledge to larger numbers.