Do I have to have a showing saddle to go showing?

Absolutely not. There's nothing saying that you've got to have a showing saddle to go showing. What a showing saddle does is it's straight cut and you can see the shoulder of the horse. It's basically a nicer shape and shows off your horse better. It just looks nicer, but there's nothing saying that you can't go showing in your general purpose saddle, your jumping saddle, or your dressage saddle. As long as it fits your horse, there's absolutely nothing stopping you from getting out in the ring and having a go with whatever saddle you have.

When I was younger, working hunter saddles used to be a popular for working hunter classes. They are straight cut saddles, but with a little bit of a deeper seat and a bit more knee roll, better for jumping. However, in recent years, I've noticed that not so many people are using those now. People are actually using jumping saddles in the working hunter classes. Even the professionals. Again more evidence that absolutely not, you don't need a show saddle to get started in the show ring.

For younger children, if you are looking for something that's straight cut but you don't want to splash out on an actual proper showing saddle, there are saddle pads. People use these on ponies up to about 12.2hh, they are a cheaper option for getting the straight cut look. Also remember that you can pick up second hand showing saddles at very reasonable prices.