Easiest way to reduce body & sleeve width?

My kids are 6 and 4 and both their measurements fall into size 5. I had a muslin done (pic attached) and while the finished shirt is fine for casual wear, I need to make the body and sleeve a bit narrower as the shirts will be worn under suit jackets for a wedding. The shoulder fits them perfectly, just a bit too wide. Rather than messing up blending sizes on the pattern, can I just take in the side and sleeve seams? So attach the sleeves as normal but do 1" seam allowance for the sleeve and 1.5" allowance on the body. Then trim and finish the seams. Will it work or is there something I overlook with this approach?