Good rain gear can make or break a trip, and if you are cold and wet you won't want to come back, and in many cases it can be life threatening. Many parents tend to skimp on this essential piece of gear, and that is a mistake.
You will want a two piece set that is tough and breathable. It should not be plastic or PVC and ponchos are best left on the store shelf. These cheaper items will rip, and that is bad.
The higher end items will be made of Gore-Tex® or a similar fabric, but waterproof nylon rain suits will work well, and cost much less. The higher end suits will run you $200-$500, but you can get a good suite for something around $80.
You rain suite can also act as a year round outer layer to cut wind, and reduce the number of other heavy items you need to carry.
My year round outer shell is made of Sierra Designs G3 fabric. The initial purchase hurt the wallet, but I recall sitting on the ground in tornado driven rain once, cooking casadias in my backpacking oven, dry and warm with a silly grin on my face, enjoying every minute! It was worth every penny, even if it is a tad heavy.
My boys & I also run a set of Sierra design backpacker's rain gear. It compresses well, and if you are not expecting extreme weather works very well. This is adequate for most scouting trips.