Hi chennai,
From what friends and family have told me temps right now are between 16-20. Usually July in Calgary is in the 20s. Depending on rain, winds etc. It could go down as low as 12 but then go up to low 30s. Just depends on the year. The evenings are a little cooler though. If you're coming from India you may find it cool for the first bit until your body adjusts to the temp. I don't know what prices are like in India. I would say Calgary is a little pricey but there are places you can go for cheaper clothes etc. We have big grocery stores that have weekly sales. Your best places to go for food are Safeway and Coop. For household items, clothes etc you go to Walmart. They're everywhere. Summer is usually considered to be mid June to Sept. but the weather changes quite a bit. You'll really enjoy your first fall there with the colour changes in the leaves. I would say bring a light jacket with you. But you can find everything in Calgary. If you're looking for certain India ingredients for food you'll find them in any of the stores. You will have to tell me what ages your children are as we usually go by age group since in the school systems in Alberta we dont use things like 6th standard. I'll be able to tell you more about what grade they'll be in then.
As for when you can admit them.... School year begins in September so that's when the big admissions happen or they happen prior to the school year. However, our schools won't turn you away if you have them admitted in January. I actually had kids start in my classes as late as May (school year ends at the end of June). Schools go by the area you live in.