Children tend to repeat what they hear, so I'd guess they must be hearing "guess what" a lot. I guess it's a good conversation starter especially if you're teaching them some fun fact or sharing a pleasant surprise.

Or maybe it's a call-and-response thing. When someone says "guess what" the listener probably replies with "what?" so it's almost like a game.

Depending on their age and development, sometimes they say things and don't really know what they're saying. My friend has a five year old, and I guess I must have been sharing a lot of stories with him because one day he busts out this gem: "When I was your age, triceratops has three horns."