Eoghan,
how will you measure the speed? I think it is more likely that you will use a fixed height and drop two objects together and see which one hits the ground first, in essence comparing time.
Note that using the actual formula pushes this activity very much to the upper end of primary.
(Maths has Explores the relationship between time, distance and speed, but as a j).
By using different objects, the variable the children will be investigating is likely to be surface area. If they are also varying mass, then comparing objects will be quite difficult.
This Gravity resource from Engineers Week structures the inquiry slightly and might make it easier to ensure that the learners are building up their knowledge.