Hi Eimear,
I think if you want the children to actually time something falling that you will need to give them objects with a lot of air resistance, otherwise, it’ll all be too fast.
Or, you could go to ramps, and have them at a very shallow angle and have objects slide down. If you have marbles that roll, or toys that roll, then a shallow shallow ramp may work.
Measuring time appears in the Maths curriculum for 3rd/4th class- would you introduce it much earlier? [compare, approximate and measure time using appropriate units of measurement.]