Let them know who you are, what you believe and let them see you being a good human. Answer questions when they ask. Really all you need to do. Will the parents be pissed if you answer questions about yourself honestly?

I like the science kit and magazines ideas.
There are those monthly science kits you can subscribe to. Expensive but kids like getting stuff in the mail. One experience ready to go without alot of adult help seems like a genius way to get them to dabble and think.
Cricket media has some good science magazines aimed at different age groups and literary and historical themes as well. Lots of stuff to encourage kids to ask questions. I like them better than some of the other options because they seem little more engaging. More pricey than some of the other kids magazines. [shop.cricketmedia.com]

How old are they? If they are old enough you can tell them that its o.k to be a catholic if they're content to be a virgin well into their mid 20's.

Not sure that it helps. Don't forget, the guy who came up with the "Big Bang Theory" was a catholic priest: [en.wikipedia.org]

Maybe mention that being a good person has nothing to do with the existence of a god. Then let them figure their own path.

Take them to see Neil Degrasse Tyson.

I look for science happenings around town such as the free telescope night at the local science museum where folks come out & look at the moon/stars/sky & get a bit of astronomy, the "nocturnal animal nights" at the local wildlife center where you get a guided tour of the area ($5 per person), even the local art museum has a ton of science & math then & now (perspective in art, creating colored glass for stained glass windows) not to mention they usually carry things like acient Egyptian funerary art & mummy (resurrection, mummification, how silly), reliquary (kept the bones of saints...saints bones was big business, lots of money...how silly), etc. Sonetimes just gently pointing things out gets the wheels turning

I'm also the "grandma of books" & I usually get my grandkids a fun book & an age appropriate educational book. Who says no to books? They are quiet toys...what parent is going to argue w/ a quiet gift?

Do not say anything. Buy them science books and kits. You cannot take over their parents role. They will have to come up with their own conclusions regarding religion.

Where was this if Catholic school may explain a few things