Only 1 child can be linked to an account. However, you are welcome to create more than 1 parent account. You'll just need to use 2 different email addresses.
The reason only 1 child can be linked to an account is to ensure that each child is matched with scholarships they are eligible for.
Each child may have different GPAs, interests, intended majors, and skills. This information determines which scholarships students are eligible for (and thus, which scholarships appear in your account).