Aww I think this situation will actually benefit you
Although it'll seem difficult at first, don't panic. Kids probably won't even notice if you get a little frozen, as they're concentrating on their pony and riding, and will only be listening to you like 80%, and if they notice and giggle now and then, they're only kids, random things amuse them, it's probably not even you they're giggling at! If you get a little stuck, take a deep breath. If you forget how to explain something, think how you'd do it. If you get really stuck, go up and show them (like, using leg aids for example, take the pony to the middle, and physically press the child's leg where they need to be pushing the horse to ask for the response, and you can use minimal words because you'll be showing them you just need to say "Look, as you press you leg, your horse knows to move in that direction. Now you try by yourself and see what the horse does" Or something like that)
Don't over think it, have a laugh, if you get totally stuck, just say something dumb like "Oh goodness, I've forgotten what I was saying, somebody must be stealing my thoughts" and do it in a really animated way, kids lap that stuff up, and do a big laugh afterwards and they'll laugh with you.