I have done both, cooked from scratch with a unit, and given out premade cakes to ice. Have used packet mix at camp when cooking on a fire chocolate cake in oranges - more time is preparing the oranges!
90 mins is not a lot of time when you have girls drifting in 15 mins into the session, girls you are having to teach some basic skills to - like weighing, cutting, creaming etc. I would say 50% of my units girls NEVER bake at home so we are covering a lot of skills.
Clearing up is also something that takes for ever, most expect a dishwasher to sort things out. It can take them up to 20 mins to actually clear up and I refuse to do it for them.
We share our kitchen with 3 other groups so we have limited space, plus a domestic size oven which is not the best.
We also only have 2 leaders and 30 girls - I am sure you can do the sums - we have parent helpers but they can be a bit hit and miss as to how much help there is, last one we had in only helped her own daughter all night even though I asked for her to do other things, this wasn't baking.
Also we do not have a huge amount of equipment - and bake so infrequently its not worth investing, scales, bowls, spoons, measuring spoons, our unit is split into 5 patrols, if we want everyone to do it at the same time I end up with half my kitchen in the car!
As I said I have done it, I have done it using various methods - weighing it all at home - may as well give them a packet mix, only doing a couple of groups a week - not the best if making something for a specific reason.
I tend to look carefully at what we are making and see which is the best way.
As for cooking with 5 year olds, have also done that, would not be a group of 25 of them and 2 adults I can guarentee that. Or if it is then I will happily let them loose on my guides!