Hi.

I taught preschool last year. A few websites that helped out were:

1) perpetualpreschool.com (specific activities under themes and also divided into headings "science" "songs" "books" "art" "games")

2) enchantedlearning.com (arts and crafts ideas...lots of templates)

3) dltk-kids.com (crafts that enhance specific skills, ideas for games and activities)

Hope this is what you were looking for. For mixed age-group classes I would suggest allotting time where you have the children work on different tasks (such as playing with blocks, or reading books), while you are working with a small group at the table or carpet for their age-appropriate craft/activity.
CB