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.