I purchased the Regular Peewee for my son who is 6 for baseball.  He is 47 lbs and 48 inches tall, a tall and thin build.  He usually wears a size 4 or 5 boys' brief.  These briefs fit him very well until I washed them (granted I did wash them on warm and dried on medium).  They still fit, but seem a bit on the snug side.  He has not complained about the fit at all, however, so they should be okay for this season.  If your child has an average waist for their age, you might want to buy a size up.  The briefs seem like they are well-made and will provide adequate protection.