How to say no. As the only thing you're likely to hear for the next 10 years is 'can we get a pony?' 😆
It'll be very low pressure. She'll be introduced to the pony who she will be riding, go over some safety rules for interacting with the horses and ponies and be shown how to pet them, probably be allowed to give her pony a small treat and told who is safe to approach and who not to.
She'll be fitted with a hat if she doesn't have her own and they'll get her on and teach the basics. How to hold the reins, how to get the pony to stop and how to ask him to go.
They'll practice this a few times on the lead rein and then start on how to steer left and right. She may be allowed to walk round off the lead rein at this point, depending on the temperament of the pony.
We usually end first lessons by putting them back on a lead rope with either the instructor or an assistant/older child as a runner and give them a little try at trotting if they want to, so they get a feel of it.
They usually come off beaming from ear to ear and, if the pony doesn't have another lesson right after, are allowed to walk the pony back to his stable with an adult who will then explain the names of the tack and what its for as they take it off the pony. We finish with a pat and a treat for the pony.
She'll need to wear close fitting clothes, nothing loose and flappy. Long sleeved T shirt and leggings would be perfect. Have a jumper over if it's cold. Her shoes need to have a bit of a heel and come up over her ankle. If she has mid calf boots for winter, those would be good. Don't put hair up or use clips as it will make the hat uncomfortable, a low ponytail is ideal.
For yourself, wear sensible shoes as the ground will likely be quite muddy in places and most schools are sand so if she wants you walk with her, you can. Coat you don't mind getting a bit mucky as horses are curious creatures and can be grabby sometimes.