How to break the news to little kids?

I have three baby cousins who I completely adore and who adore me. The oldest one, he's 4.The middle one, she's 3. And the baby, he's about to be 2. I used to always play a little rougher with the oldest two. I couldn't with the third because he was still a small baby. Now, I'm 13 weeks and 5 days pregnant. They have no idea yet. When I go over, they always want to play. And now the baby, who's not such a baby anymore, wants to play too. Sometimes they would like play tackle me and tickle me and all that good stuff. But now obviously I have to be a lot more careful. How do I explain to them that I need to be a lot more careful because I'm pregnant?