I think you should tell them right away. I was going to agree with Judy and say that the first couple of weeks can be an adjustment and it feels crazy and then gets better...but, then I noticed you said your kids are 2.5 months and 18 months! Wow! You are brave to add anything to that let alone an extra two. Definitely you should follow your gut and not get yourself into a situation where someone might get hurt or you are a total stressball. Good luck to you