Hi! CogAT is useful for gifted exam ( gifted schools: Edison,Bell, Pritzker etc).
If by SN you mean Skinner North ( classical exam) the kiddo would need to know addition& subtraction up to 20, and to read 4-5 letter words.. at least this is what it was last year. For kindergarten ,gifted&classical exam are taken together, but you’d want to focus on the classical part of it to get into SN.
Hope it helps! Good luck!