You may wish to include a UV LED in your Science Play and Research Kit. Nowadays you can find inexpensive key chain lights that include a visible light LED flashlight, but also include a red laser LED and UV LED. The red laser can be used to demonstrate the Tyndall effect, and the UV LED can be used to demonstrate  a lot of stuff that becomes visible under UV light. I found this black light (UV) LED strobe light on clearance after Halloween for around $2.00 USD:

Here’s a better picture:

(Source)

The beaker filled with cloudy liquid is actually a few drops of milk in water that I used to demonstrate the Tyndall effect with a red laser. The next two pictures show the beaker with the UV LEDs off and then the UV LEDs on:

There’s a lot of stuff that will fluoresce under UV light:

Milk (as demonstrated in this article)

Urine (that’s what those trippindicular black lights you can buy at Petco are for—but you can drag those old black light posters of Jimmy Hendrix out of the attic if you want).

Soap

Sunscreen

Lemon juice

Tonic water (the quinine flavoring in the tonic water)

Petroleum jelly

Chlorophyll

Antifreeze

The security strip on US currency

(See my source for more stuff that will fluoresce under UV light, and also take a look at the black light photo gallery)

You can perform a simple demonstration of UV fluorescence using a Q-tip, some colorless liquid laundry detergent, and watercolor paper. Pour a small amount of colorless liquid laundry detergent into the cap of the bottle to use as an ink well. Dip one of the cotton swab ends of the Q-tip into the laundry detergent and use the Q-tip as a pen to write a message on the watercolor paper. Wait until the detergent dries and then shine the UV light on the message to make it visible.

I used watercolor paper since 8.5 X 11 in. 20# computer printer paper will itself fluoresce under UV light and possibly obscure the message. Watercolor paper is off-white making it easier to view the message:

This is a beaker with some laundry detergent dissolved in 250ml of water with the UV LEDs off:

This is the same beaker with the UV LEDs switched on:

This is the message illuminated on the watercolor paper using the UV LEDs: