What is considered a high blood pressure and what is considered a low blood pressure?

There is a range. Generally the diastolic (lower number) must be over 90 to consider it hypertension. The range for the upper number (systolic) is broader but is a problem when over 130. The low end is about 50 diastolic but tolerance for this varies.
Varies with age/sex. Age, growth & gender alter the upper/lower levels considered normal for BP for kids. Systolic hypotension would be considered <60 for newborns<70 the 1st yr , <70+2xage up to 10 then <90 thereafter. Systolic hypertension is age, gender & length/height related in kids. Readings >~100-107 for 1-4yr progressing to 125-135 in late teens. Single values are less reliable than patterns.