How to keep a small pool clean?

Inflatable children pool - simplest way to warm up water and keep it clean over a long weekend.
We got out of the inflatable children pool for week-end. Its lovely and hot here, and so I've already been leaving the water to heat up throughout the day, then children play when you look at the water when you look at the questionable mid-day. We filled it on Saturday early morning, covered it Saturday night, and played inside once more on Sunday. We pointed out that the areas associated with pool and the share toys had a slightly slimy surface on Sunday. What's happening, is this the origins of algae? I emptied, cleaned and refilled it today.

Do I need to put something in the water to prevent this happening, or just empty, clean and refill every day? I estimate it has about 100 gallons of water in it (its about 7' x 4'), and I do have chlorine tabs meant for my 60 gallon emergency drinking water barrel. Should I put one of those in for future weekends, or something else?

Incentive research question - we covered the pool with on a clean roll of painters drop fabric - clear plastic, but textured so its semi-opaque. Would within the share with plastic through the day make it warm-up faster, slower or make no distinction? The share is sitting in full sunshine all day long.