Sodium thiosulfate and Hydrochloric acid
There's a nice demonstration of the experiment performed by Annis Hapkiewicz.

She uses a 0.15 M $\ce{Na2S2O3}$ stock solution.  She prepares 5 beakers with 50 40, 30, 20 and 10 mL of the stock solution filled up to a total volume of 50 mL with destilled water.

This corresponds to thiosulfate concentrations of 0.15, 0.12, 0.09, 0.06, and 0.03 M.

I couldn't figure out the concentration of the hydrochloric acid used, but the times for the appearance of the sulfur cloud (disapperance of the cross) were given. 

The range is between 20 s and 5 min.

I'd probably go with the 2 M HCl for safety reasons and perform the experiment with the thiosulfate concentrations given above. Check the times and adjust the concentrations if necessary. 

You don't want it to to fast to be measured by the kids, but you don't want the highest concentration to take half an hour.