While the U12 are transitioning and could make do with just a pair of good SL skis, aim to have one pair of SL and one of GS skis by the time they're 12. For U14s, you may look at an SG pair or rent one when needed. Read more about Ski Boot Fitting and see Ski equipment for U10-U16.
The only reason to get two pairs of either kind is to save you some saturday nights, if you have to tune before a race. If you do get two pairs, make sure they are identical and you use them equally for racing and training throughout the season: do not keep a "race" pair, which will always feel more sharp and different from the other one.
You have to start sharpening the skis more frequently now and keep them waxed with basic all temperature all the time, to provide a consistent experience. You don't want the the skis to feel all of a sudden too sharp or too fast on race day - keep them sharp and fast all the time.
Since the kidz are now building precision, it's time for your tuning to get more precise as well, so more tools and time will be needed.
1 degree base bevel and 2 degrees side is good up to U14. A strong U12 can be advanced to a 3 degree side bevel with the coaches advice - see Ski Tuning - edge bevels. Don't rush this and do upgrade all his SL&GS skis at once, to keep things nice and consistent.
Strong U14s could look to change the base bevel towards 0.5 and keep the side bevel at 3.
Speed skis (SG and DH) are different... If you have them, keep them at 1 degree base and 3 side and move to progressive base bevels when warranted. It actually is a safety issue here, so be sure to talk to a coach: a too sharp base bevel makes the skis too grabby at speed.
For wax selection, you can get more specific now and use race versus training wax. Make sure to use the race wax for training as well often, to get them used to that feel. I don't think there's a reason to get into the really expensive temperature-specific waxes yet, up to you... use the moneys to get better tuning tools instead. The only thing is to maybe use a special "cold" wax for colder than -10 Celsius - see Choosing ski wax.
Besides the mandatory helmet and goggles, as they progress and get on courses more and more, it's time to start fitting them with good shin guards, hand guards and mouth guards. Your coaches will ask for those.
Other items would be race gloves for GS (with protected knuckles etc). Back protectors and tight fitting body armour is good as their speed goes up.
For tuning tools, you are now building a reasonable collection, as you move towards U14, where you should have:
You are now hot waxing - keep some rub-on and/or paste in the backpack, just in case you forget one day or notice a ski very dry on the hill. Use a good all-temperature cheap stuff like the Hertel SuperHotSauce for training and add some all-temperature racing towards U14.
The only issue with a good all temperature wax is that it's often harder than the coloured temperature-specific varieties, so you could look instead at the basic tri-color Kuu. I enjoy the freedom of not caring about the next day's temperature, though.
When you get tired of brushing out by hand, look into getting a pair of two horse hair roto-brushes, one for race wax and one for training/hot scraping. I also have a boar hair brush, which feels just like a horse hair and a brass/horse hair which I use for cleaning... but this is more like a hobby now, heh.
See more about Deciphering Brushes.
As a process, it's more intense and more frequent than the U10, and even more frequent and more precise towards U14 - the idea is to keep the skis in shape at all times and keep them gliding well consistently throughout the season.
Get the ski profesionally tuned at the beginning of the season, including a base grind and re-setting the edge bevels (to 1 and 2 or 1 and 3 respectivelly). Then just maintain them weekly: lightly run the file on the edge, followed by the 200 diamond stone, one pass on the base edge and 5-10 passes on the side edge, followed by the 400.
Every week, check the sharpness of the edges. If still sharp, leave them be for training. If decently sharp, use just the diamond stones.
Weekly, Repair anything you see wrong with the base, edges or top-sheet. Rub on every day and hot wax every other day, as a minimum.
For race day, make sure you start with the smooth file and finish with the diamonds, up to 600, for a great grip. Then hot scrape and finish with the Fluoro race wax for a better glide.
If you feel like doing more than neccessary, take a peek at the next age level Ski equipment and tuning for U16 and U18.
As you move to U16, the tuning will only get more and more demanding. It is now the time to teach your U14 to tune his own skis, at least for training. Start with hot waxing/scraping/brushing and then maybe let him use the diamond stones when he feels the ski doesn't grab in training and divide some of the work.
When the kids move up on ski sizes, you can help them by setting the bindings forward. Some, like the Atomics are built to easily go forward/central while others are not - one reason why I prefer Atomics. Forward is also good on ice while central/back works on fresh or mushy snow. Read more about Mounting the ski bindings forward or central.
Start fitting the kidz with shin guards at U12, because the stubbies will chew up those pants.
Read on for even more Ski Tuning Ideas.