I've seen answers that are just AHHHH! or AAAHHHHH! if the character is frightened, but what if the character is excited? I've also seen people put an exaggerated "SQEEEEE" for excited but I don't really think that's appropriate for the story I'm writing.
You could just have the action narrated:
Webby squealed with girlish delight, "We can do actual Magic."
Ultimately though, Onomonpedic exclamations are generally fine in books so long as they can be distinguished. "AAAAH" is a very specific sound that a person makes in certain circumstances. There are different reasons to squeal: Are you making an overjoyed squeal because you get to meet the actor who plays your favorite character on your favorite show or are you a pig?
Excitement could be "WHOOOO" or "HELL YEAH/F___ YEAH (you get the picture - cuss words)" or "YES!" I'll admit that "SQEEEEE" made me laugh a little bit.
When people are excited, they shout a celebratory word, and/or their volume just rises in general. Showing body language could also add to your sound effect to make its purpose more clear (bouncing on the balls of your feet, rubbing your hands together, looking all around, smiling, etc.) Think of what you do when you're excited and translate it, maybe?