I like to have about 4-5 pairs of jeans (different washes for variety) each for my 2 school age kids (DS1 is starting Grade 2, DD - kindergarten). They are both very rough on their jeans so I like to have a lot of pairs so each one lasts longer. They each also have 1 pair of khakis and DD has a few pairs of various colored leggings (capri and full length) to wear under dresses.
DD has oodles of dresses but that's because my mom and I sew her dresses and we each go a little crazy each year making cute dresses. Each kid has at least 6 short sleeve shirts, 4 solid colored long sleeve tees or thermal shirts (great for layering under the SS ones), 2-3 sweatshirts and 3-4 sweaters.
As for winter wear, I buy my 2 school age kids separate parkas and snow pants. It often gets cold here before the snow starts to fly so they don't need the snow pants right away. I also don't have room in my front entry closet to have 2 full snowsuits for each of them plus a warm parka for when it's cold but there's no snow. They each have a pair of well-insulated boots (the kind with the pull out liners are fab, they dry faster), 2 pairs of washable, well-insulated mitts, 2 washable toques and a neckwarmer. They each have a pair of indoor runners they keep at school and a pair of outdoor ones for when there's no snow on the ground and a pair of rubber boots for spring thaw/rainy days.
I know you said you're on a tight budget... can you check out yard sales in your area? The other weekend, I found 3 pairs of jeans for DD from The Children's Place in like new condition for $1 each! I also found some new t-shirts for her for 25 cents each. Can you do a clothing swap with a friend? We're saving some money on winter gear this year by swapping with a friend. We both buy our children's winter gear from
MEC and my DD will fit into her DD's outgrown winter gear and her DD will fit into my DS1's outgrown winter gear. We both buy the kids' gear in gender neutral red so it works great to swap for either gender.