1. Get a second opinion.
2. get more info.
Having a brace later might mean all sorts of different things. It might mean one of those round-the-head traction jobs, or having 8 teeth removed to make space BUT...
if the dentist is right and it's just thumb/blanket sucking that is pushing those front teeth out, then my guess is that if it DID need a brace, it'd be a daytime only or nighttime only removable, top teeth only job. Which your child might well consider is a good trade off for several years of happy sleeping.
One of those top-and-bottom, fixed brace with elastic bands connecting to a night time jaw moving machinery... no, that might not be considered a good trade off.