For gear I always like to try to go into a track/store and try things on, what might be comfortable for one person may not be for the other, especially true when it comes to kids.
If you can’t get to a track that sells parts/gear, most dedicated motorsports/auto sports shops will have gear you can take a look at, although it may be more geared toward adults.
I know there are a few topics about online stores but in general my go to are
Fastech and AKR are in Nevada so it takes a few days for things to come, whereas Ts is in Bushnell and comet is in Indiana. Comet and AKR especially are very helpful when it comes to answering questions, and could definitely help you out on specific sizes if needed.
I can’t speak to what brands fit kids best, But generally OMP, Alpinestars, and Sparco carry quality suits and gloves. Arai is normally a favourite when it comes to helmets, and Bengio and Stilo are great rib protectors. I can’t speak to the best neck protector so maybe someone else can chime in. Regardless I would recommend trying on a few pairs even if you go to your local track and just ask to have your kids try someone else’s on, for safety gear is important for it to fit nice and feel comfortable while also providing ample protection for growing bodies.
Hope this helped a little and let us know if I need to add/change something I stated as like I said I’m not too familiar with kids sizing.
Id recommend getting CMR rated helmets. These are specifically made for kids. Snell rated helmets generally are going to be heavy for the necks of the little guys. Bell makes a really nice CMR helmet. Zamp makes some lower cost CMR helmets also. Stilo has their flagship Karting helmet available in a CMR also.
It’s really hard to get feedback from kids on how gear should really fit. To my kid, a helmet is a helmet. The fact that it is there dominates any sensation of how tight it should or shouldn’t be. Thankfully, she loved it right away. The rest of the gear is usually pretty adjustable, and you want a lot of adjustments anyways with how fast they grow. How something fits one month may change the next.