I'm quite good at pony parties if I do say so myself - pity you're not near us.
With enough leaders you can do pretty much anything, after the prettiest/glitteriest pony competition (they had half an hour and lots of help to paint hooves and decorate ponies with glitter gel) we had a mini-gymkhana and then a treasure hunt. Instead of party bags they had to ride round looking for clues and find their own prizes (this is because I forgot the party bags but had loads of THINGS for the children).
I also forgot food and cake, but that's why God invented McDonalds?
your description sounds great - i'll certainly make some calls. by meatier, well the city farm option really just gives each child a very short stroll (not the technical term i'm sure!) on a little pony. the upside is you can invite 25 kids and each can have a turn, the downside is that my dd wouldn't get as much personal horse-contact as she'd like.
The best thing to do is ring round all the local stables and ask what they do. At ours we do a fab party package, where everybody grooms and pritifies a pony,(with lots of glittter spray!) then has a ride then tea then home. One of the main attractions for the little girls is the gang of big girls who help them and lead the ponies - it tends to turn into a mutual admiration society! What do you mean by "meatier"?
We are a non-horse-riding family in SW London. DD (6) has ridden once, and is obsessed with the idea of a birthday party where everyone gets to ride. I know that Deen City Farm in Morden offer rent of a party space where you do your own thing and the children can each have a brief pony ride, but I think she'd like something a bit more meaty. Can anyone suggest a riding school in Surrey - i'm thinking Epsom type area - where total beginners can enjoy a horsey experience without me spending a total fortune?