OK-- Not so sure it is baby safe... Seeing as it uses type 1 industrial RTV silicone... but OK.
Needed materials:
1) Vegetable Glycerin
2) Corn starch
3) Type 1 silicone bathroom caulking (I used clear, but any color will work. NEEDS to be type 1 silicone though. I have also successfully used high temp automotive gasket maker. So the secret sauce is that it is type 1 silicone. The kind of type 1 silicone does not seem to matter.)
4) A small amount of water, About 3 to 4 mL per 100mL of caulk
5) Optional- Food coloring
6) Recommended- rubber gloves, disposable mixing container
Process:
Combine water, food color, and vegetable glycerin. Stir in corn starch until it reaches the consistency of heavy cake frosting.
Put on rubber gloves. Add type 1 industrial silicone, and mix thoroughly. (Depending on how firm you want the foam, go between a 1:1 mix of silicone to cornstarch mix, to a 1:2 mix of silicone to cornstarch. More cornstarch will result in a softer, lower-density foam.) Once mixed, the clock is ticking. (Type 1 silicone is activated by the water) At first it will 'sag' when left to stand, but as the silicone sets up, it will become more silly putty like, and will hold its shape, but you dont really have a lot of time. Using a mold would be benefical. (pam cooking spray, or just a light oil would work well to prevent sticking. More of the glycerin would probably work too.). Shape appropriately, and let cure for about 1 to 2 hours.
Once set, it will be very firm to the touch. However, because the starch was wetted completely with the glycerin, it will wash out of the rubber when submerged and gently squeezed. Depending on how thick you made it, it might take awhile to get it all washed out.