I think there is a huge difference between a 2.5 year old and a 4.5 year old, so I might consider splitting the class differently. A 4.5 year old deffinately does not need a parent doing in with them. At 2.5 these kids are still babies. I would consider making a 2-3.5 year old class where parents assist and then a 3.5- 5 year old class.
I am not a huge fan of the "lets pretend we are dinosaurs and imagine this and that" type teaching style either. Once in a while it needs to be thrown in for a short time, but I like to use physical objects and props more. Stretching has the potential for being the most boring part of a preschool gymnastics class, but I think it is one of the most important parts. Not necesarily because they need a good stretch relative to the activities they will be working on, but more so to teach them basic gymnastics terminology and body positions. You can make it fun!
Stretching ideas:
This is when the imagination might come out a little bit..
1. Sit in a pike and make a pizza- "roll out the dough by reaching from your knees to toes, rub the sauce on your belly, sprinkle the cheese on your knees, cut the pizza in half- now your sitting in a straddle"
2. Straddle stretch we make rainbows- give the kids a pile of different colored objects by each foot. Bring a red object from one foot over the top and put it by the other foot. Repeat with different colors
3, incorporate a song like heads, shoulders, knees, and toes or row your boat (to be done in a pike strectch aka your boat of course

)
4. Incorporate letters into stetching= V-sit; letter T is "side middle arms" letter O is "crown arms", letter I is a "finish"
For Equipment and Skills:
Incorporate props in again
1. walk across the beam carrying different objects, stepping over objects etc. One week it could be letters, another week colors, another week shapes. I think parents like this practical stuff!
2. Bars- hold something between your feet while you swing, balance somehting on your knees while in a tuck hang
**If doing any front support skills, pullovers, or rolling actions around the bar I would suggest cutting a piece of pool noodle to tape around the preschool bar so it doesn't hurt their stomach. The piece should be just as wide as their hips, but still allow them to grip the actual bar
3. Vaulting- make a game of who can make the loudest noise when they jump on the springboard; play hopscoth; practice running differnet ways (knees up, kicking bum etc), jumping on one foot then the other
4. Floor- preschoolers heads are much bigger than their body, so be especially careful! They should also not be doing any bridges.
Set the expectations for parents as far as how much they need to participate and help (i.e - Mom's do not come in your skirt that you wore to the office today!). Also, educate them on proper ways to spot their children and also what is realistic for preschoolers to be doing vs not doing.
We have a lot of the Kimbo CD's for the preschool music.
If you can find some cheap big puzzles you can incorporate that into your circuit for hand eye cordination. It also helps to slow the circuit down a bit. So you might have one puzzle at the beam (walk across with a piece and put in the puzzle on the other side). Then the child moves onto the bar, where there is a different puzzle. Take a turn on the bar and then put one piece in this puzzle and move on etc.
Do you have a parachute? That is always a HUGE hit!
Got to get going, but hope that gets you started!