I have 2 DD's at home (DS at school) and DD1 goes to preschool 3 days a week. Currently on mat leave.
DD1 will be old in the year at school this September (birthday in September) and have lately noticed she seems really bored on her days at home. A lot of her friends we used to hang out with on my days off went to school this year as she was the youngest of the group and although we have other friends we see, she is now the oldest child by a long way and she is a bit fed up I think.
She misses her brother when he is at school and her baby sister obviously takes up my time and does rather dictate the day, in terms of naps etc. She is also about to crawl and so will become a whole new level of irritating to DD1! 
When I go back to work, I will still be off on Tuesday and Friday and I am thinking of starting a preschool session on a Friday too (currently goes Tuesday- Thursday) but it will mean she won't then get a full day with me at home.
Would I be awful to do it? She loves preschool BTW as she has loads of friends there, most of whom will move up to school with her. I just feel guilty that she will miss the time at home with me and DD2 but then, she doesn't seem to enjoy it much anyway at the moment!