Petrie Cast for 4 yr old

A friend of mine contacted me about her grandson who just turned 4. He is having surgery the end of the month at Shriners Hospial in St. Louis due to a hip problem and will be in a petrie cast. I thought it was the same as a spica cast but I'm not sure as I have never worked pediatric ortho (or any ortho for last 14 yrs). I am thinking a spica cast covers to the waist but the petrie cast only goes to the thighs?

Since I sew, she asked me about altering/making him some pants with velcro or snaps on the inner and outer seams and doing the crotch of some boxer shorts the same way. I need to get them done by the 20th since they are coming here (2 hours away) that day for a belated birthday party for him.

He wears a size five but if the cast covers the waist, wouldn't I need to make them much larger to account for cast room (width)? If it only goes to the thighs then only the legs need to be larger so size five but with really flared legs?

Since we live so far apart and I work full time, I only want to do it once and not have to re-do them later b/c they were too small. I don't imagine he'll be up/about much so if too large, might be ok. OTOH, if he's able to get up and move around, I don't want them falling off.

Can anyone offer me any advice?
Thanks, IndianaLPN