I just had DD 5 days ago. DS has been a handful for the last month or so. Not listening, whining and crying about EVERYTHING, etc, but he's about to turn 3, so we've been expecting it. His behavior has very little, if anything, to do with DD's arrival. His daily routine has remained untouched and we give him about the same amount of attention as usual. Today has been awful and DH and I are considering taking DS to preschool 4 or 5 days a week; he currently goes 3, but we pay for him to go every day. Is it terrible to take him more often? We are both just feeling so overwhelmed and I feel like it may keep him more grounded? Does that make us terrible parents? To dump him at school more often for our own sanity?
Re: Preschool dilemma
I would definitely take him for 5 days/week. We also pay for our boys both go 5 days/week and I do not necessarily take them every single day but they do completely fine on the weeks they do go the full five days.
My kids have fun at school and they keep them on a schedule and really busy. I would not think you are terrible parent for taking him more often. I say do it!
TTC since 11/05...ectopic pg 4/08...early m/c 6/09...BFP 10/5/09!

Nora B...June 15, 2010...8lbs, 8oz...Med-free birth!
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Malcolm...September 21, 2012...8lbs, 6oz...Another med-free birth!
I think you should wait a little bit before making that decision. You say your DS's change in behavior has very little to do with the arrival of DD. Even though his routine has remained the same, and you feel you are giving him the same amount of attention, toddlers are very attuned to changes. He knows something big has happened in his family. He has probably known for the past couple of months. As mom has slowed down, the time spent with him may not be different in quantity, but perhaps quality. I know I play differently with my DS now that I am tired and sore. I know he recognizes new items coming into the house that he has not seen. A new baby is a huge change for a family, but imagine what it is like for a toddler who doesn't fully understand.
So, while I don't see a problem with sending your DS to preschool for 5 days a week, give him a little time to adjust to the changes at home. At this time, it almost seems like a punishment for him having a normal reaction to change. Definitely do not feel bad about sending him to preschool for 5 days a week, though!!! He will love it!