I get prodeals since I work in the industry, so I don't really shop retail much. But, I know they're already going on because I've been getting emails from different companies. What do you need?
I would try to figure out what you want first and then just google shop them around. In the past, I've gotten some good equipment deals from: Sun and Ski REI Skate Pro
Apparel I have not noticed much difference buying in-season and off-season. I am sure deals are out there, but I am always scrambling to buy my son new stuff each season and usually happy with the deals I find in-season. Never have used one store; again just google shop what you want and sometimes that leads to certain stores having some good deals.
At 10 years old, I'd look for a new board and bindings. I can't really help much on snowboard hard goods, but if you ever decide to split board, I can get you some hella deep discounts.
I have no doubt that retail stores and such will have big markdowns. I just haven't seen that typically on some of the more obscure online sites that always seem to be pushing promos, etc. But I definitely defer to you since you're in the biz.
I always buy my daughter's clothes (online) in the next size up in late March/early April. Last year Roxy had such good markdowns that I bought her next two sizes up, so I haven't looked yet this year. I've also had good luck with Burton in past years. REI can be hit or miss with kid's stuff, but the last time I needed a new jacket for myself, I got an Arcteryx one from them for about 25% off in April.
I was exaggerating a bit, but we will ski 3-4 days during my son's spring break then 2-3 weekends in April. I think the best skiing of the whole year is right now since the Texans have gone home and no one is there. Plus snow has been coming later and later every year. Last year we skied into May
If you are ever in person you can shop the thrift stores. Rich folks use and donate gear...especially for kids. One season of use and tons of ski apparel in Montrose from Telluride folks. I'm sure other areas are the same....