i was told not to use anything that would seal the cement, and that the object needs to breath.
You can use the outdoor acylics that are in the craft stores. Just paint the top and leave some of the bottom open you won't see. I don't see the problem though- I painted a hopscotch on the sidewalk on the side of our home for my daughter-7 years ago. Its still there-no sealing. Check out liquitex and other main brands-ask the store/girls for suggestions.
As a rule, sealing cement as with other porous materials is generally a good idea. The reason being is that should moisture absorb into the cement with a nice coat of paint, the first thing to happen is that the paint can pop off. In chunks if it's oil or thin flakes in latex. Either way I would suggest painting with any finish you like and use coat of xterior primer as well. Otherwise, your turtle will continue to drink all the paint you could put on it.
Any kind of paint you use is going to seal it but that should not matter it will not hurt it
I believe you were misled, it won`t hurt a thing to seal the cement, so use any type of paint you want.
