Use materials that are non-toxic, even if it's safe for kids, it may not be safe for animals, reptiles, turtles,

. I'm not sure which exactly, use your best judgement.
Don't use materials or adhesives/paints that seem to come off easy in water, this may relate to what I said above.
When putting things together, it may be wise to make sure everything is securely pasted or whatever, so no small parts can come off and maybe get impacted the same way gravel does to a turtle.
Sharp objects may hurt your turtle (if you didn't already know,

).
That's all I can think of at the moment.
Hey hey hey.