Well, you could, when planting out, dig a deeper hole than you need, throw the tadpoles in, put a layer of soil on top and then plant whatever you want over that. A fish head or half a dead fish is sometimes used in the bottom of planting holes in this way as a form of direct composting, usually for food plants - as the fish remains break down, they fertilize the soil for the plant above, but I wouldn't have thought tadpoles would do the same job as effectively, although it is a way of disposing of dead tadpoles and 'recycling' them, and shouldn't do any harm. Info on this and direct composting here http://www.veggiegardener.com/use-direct-composting-in-your-garden/
I'm more curious as to why you've got dead tadpoles at all...