I found a set made by Smith & Wesson for kids. I can't remember if I got them from Midway or Cheaper than dirt. The foam plugs were smaller and the glasses fit the 6 & 8 year olds just fine.
The plugs at Walmart are a little firm. Try some softer ones that compress a little bit. If you check with your local ambulance crew the might have some that will work better. Most companies provide foam ear plugs to their crews by the handfull. The problem I ran into with kids and ear muffs is it almost always caused issues with the eye protection.
If You Heard The Shot, You Weren't The Target