Some fish, like sardines, salmon, perch, and rainbow trout
Foods that are calcium-fortified, such as some orange juice, oatmeal, and breakfast cereal
Plant milks (almond, soy, etc) often provide the same or more calcium than milk.
Calcium absorption from calcium pills is poor and can lead to development of kidney stones so I will re-enforce trying to obtain the calcium from food.
I hope this helps.
Yummi Bears has a vegetarian calcium for kids, with D3, that is allergen, gluten, gelatin, and dairy-free. The only downside is that it calls for 3 bears a day. Please make sure to brush after taking the vitamins.
In addition to supplements, I try to make sure your kids drink calcium-fortified, unsweetened almond milk daily. Also make sure your kids get leafy greens like spinach and kale which help their diet be more well rounded. Broccoli is also another green vegetable that you can choose from.
I hope I could answer your question. Best of luck.