For private tuition:
Yes. Ideally, they should be at the same age and ability level for the session to be most effective. If they are at quite different ages it's recommended that the session is extended to 1hr and 30 minutes at a minimum, this gives them at least 45 minutes of teaching each.
After School Classes:
Favourable rates are offered for children enrolled in the after school classes. Sessions are set up to cater for mixed age groups so siblings of different ages are able to attend together. Children are given individual work tailored to their learning needs and are allocated a set time to work with the class teacher as well as independently.