Yes I found my preppie (we call them trannies lol) was exhausted. Mine did swimming after school, gymnastics on a Saturday morning and started guides half way through the year. She coped fine. She had done swimming and gymnastics right throughout preschool too so it wasn't a change really for her.
She found the baby sister she acquired at the start of the year more exhausting
However, DD1 was 5.5 when she started school. A couple of my GF with younger (in that year) kids did stop doing extra curricular stuff as they felt their kids weren't coping.

You can only try it and see right?