I have a bit of experience with various Garmin units. Quite frankly, they all 'see' the same satellites and are equally accurate. You'll pay a lot more for extra features such as color screens, built-in sensors like barometers and compasses, cameras, and pre-loaded map packages. Given your stated needs and price budget, I'd say you don't need any of that stuff.
If your eyes are poor avoid the models with the smallest screens, like the eTrex.
If needing the GPS in dense vegetation or steep terrain, consider a model with an external antenna jack. I don't know an easy way to find out which models have this but my GPSMAP76S (long discontinued) does, and the lower-end eTrex models do not.
Some newer models have touch screens, which might be easier for kids and users accustomed to smartphones and tablets.
Not being familiar with current models, I've been browsing the Garmin website today looking at a small simple unit for my six-year-old. And in the same price range as you. After some brief research I've narrowed it to three units, the Dakota 10, Dakota 20, and eTrex 20. The Dakota 20 is just over $200 retail, and the others are well under. The Dakota units offer touch screens, which I think the boy will find easier to use, although the eTrex has a longer battery life and more memory. Big advantages of the Dakota20 over the Dakota10 are the electronic compass and Micro-SD card slot - worth paying $70 more?
The Garmin website allows side-by-side comparisons of discontinued models, which might be of use to you. (I see that NewEgg has many discontinued and refurbished Garmins at significant savings.)