Surfing wetsuits are made out synthetic rubber, usually neoprene, which trap a thin layer of water between your body and the suit. That trapped layer of water is what allows you to retain heat. Modern wetsuits are extremely efficient and allow surfers to get in the water just about everywhere in the world.
In the past few years Japanese "Yamomoto" rubber has burst on the surfing wetsuit scene.This materials is supposedly more "eco friendly" and blah...blah...blah.... let's cut to the chase. It allows for more thinner wetsuits and therefore more flexibility than before. I have one of these and I love it. They aren't cheap, the cost of Yamomoto wetsuits is up there with all the premium wetsuit prices. This material is supposedly derived from limestone, but it beats the hell out of me how they turn that rock into rubber.
Choosing a Wetsuit
Any wetsuit should be snug, but not too tight. You don't want your mobility restricted too much but if there are loose, baggy areas then it will be too cold, not to mention more heavy. Most brands have wetsuit sizing charts that you can consult. There may be some overlap for your size so it's good to try on at least two sized that are in the ballpark. Also try on different brands because they are all cut slightly differently. Windmill your arms around because the shoulder area is the most critical to your mobility, you can't surf if you can't paddle. Remember the wetsuit will loosen up slightly once it gets wet.
Surfing wetsuits come in no zipper, short zipper, and regular back zipper. I have had the no zip suits and I find them to be a pain to get in an out of without much added benefit. Good wetsuits seal well around the zippers. If you have a really hard time reaching a back zipper there also front zippers available with some brands.
When buying a surfing wetsuit consider the water temperature where you'll be surfing. The thickness will be described in mm for chest/arms-legs. For example a 4/3 is 4mm around the chest and 3mm around the legs and arms. Aside from thickness there are several styles to choose from.
Full wetsuit: If the water your surfing is below 60 degrees, a 4/3 fullsuit will usually do. If the water temp is usually 50 or below, a 5/3 is probably the way to go. If your surfing somewhere colder than say, the high 40's, your a little crazy but at least it cuts down the crowd. Fullsuits are also called "steamers".If that is the case you can consider some of the latest designs in surfing wetsuits, such as drysuits made for surfing and wetsuits with battery powered heater elements.
Springsuits: These should do for water temps 60 degrees and above. Most springsuits have a thickness of 2mm all throughout. A knee length with full arms or a full leg with short arms should do for water temperate closer to 60. Full legs springsuits are also called three-quarter wetsuits. The other springsuit is the "shorty", which has both short legs and arms. The shorty is usually used for water temps close to 70 degrees. Consider the thigh/leg rash you will get with the short leg version. Or suck it up until you skin goes callous.
Wetsuit Accessories
Wetsuit Hanger: A great accessory to have is a wetsuit hanger. These simple devices reduce drying time by hours. There are fancy ones with fans that move air through them but any standard wetsuit hanger that spreads the shoulders wide work just fine. Well worth the small investment.
Booties and Hoods: When the wind is blowing and it's chilly out, or if your doing tons of duck diving in cold water, the wetsuit hood is indispensable. Every surfer living near cold water spots should have a light and heavy hood as part of an essential surfing gear collection. Some of these are built into the wetsuit and some are separate. Your probably going to need some wetsuit booties to go along with that hood, especially if rocks are involved. These come in round toe an split toe.
Gloves: Usually only needed in the coldest of climates or for those with wimpy blood flow.
Wetsuit Tips
Always rinse a wetsuit out with fresh water after every use and let it dry without direct sunlight.
Rashes can develop around the neck and groin. Form fitting bathing suits will help with the groin and a rashguard underneath can help with the neck. There is also rash guard gel available.
Towel changing robes help. I have been using one for years and no longer have to worry about the "dropped towel" in the parking lot when changing.
There are only two kinds of surfers in cold water. Those that pee in their wetsuits and those that lie about it. Over time it can create a wetsuit oder. Don't use common soap, there are special wetsuit soaps you can buy that will clean all oder out.
Always dry the wetsuit inside out first, that way it will be easy to put on even if the outside is still wet.
Hang the wetsuit up when storing it. If you don't have a wetsuit hanger then put 3-4 hangers together and hang it on them.
You might want to reconsider paying top dollar on a new children's wetsuit. Let's face it, you kid will outgrow it soon enough. A new one might be worth it if you can pass the wetsuit down to one or two siblings.