Depends. There are a variety of sources depending on age and co-existing food allergies. Soy milk is a possible but 30% of cow milk allergic will be allergic to it. Goat milk or cheese, rice milk, etc. Older kids can simply be fed Tums wafers/tablets I a once or twice a day dosing based on size/need. These have enough calcium. The fluid/sugar & fat found in cow milk can be found in many sources.
Calcium-enriched. From kidshealth. Org: "Good alternatives to milk & milk products include calcium-enriched rice or soy milk (if soy is tolerated), vegan products (such as vegan cheese), & other soy-based (again, if soy is tolerated) or rice-based frozen desserts, sorbets, puddings, & ice pops." Calcium fortified orange juice can also be used although I encourage my families to give no more than 4-6 oz juice per day.
Any other. Milk, meats, legumes.
Lots of Stiuff. Fortified soy milk for beginners. Dark leafy greens like spinach, kale, turnips, and collard greens. Fortified orange juice. Sardines. Enriched breads, grains, and waffles.
Fortified juices. While there are many sources for calcium apart from dairy, sometimes the issue is getting the child to eat it. The easiest form of calcium is probably calcium-fortified fruit juices.
Several. If you like the fluid sources, soy and goat milk are common substitutes. Many orange juices & some tea's have calcium added. Tums has a chewable wafer that can provide adequate daily calcium in a simple form, & taking it doesn't bother digestion.