Hi, I am a single mom with one 10 year old girl. My ex husband and daughter's father has remarried a year ago and moved in with new wife and her 13 year old twins and 16 year old daughter. My daughter has enjoyed having step-siblings and I think it has been good for her to be around a large family. Last night, she asked if she could buy with her money a very expensive pair of boots for her step sister. How do I explain to my daughter why it isn't appropriate to spend my money on my exes step children?
I would leave the issue of spending your money on your ex's step children out of the discussion. The less you can make it about that, the better and the more well received you will be by both your daughter and your ex should he get any version of your conversation from your daughter. I would focus the conversation on spending responsibly and when and what occasions are appropriate to give gifts. In our family, we keep gift giving to a budget, so maybe you can set a limit ($15 or $20) for gift giving and discuss appropriate occasions for it , birthdays, Christmas, etc.
Also, she will probably want to give them Christmas gifts this year. When that comes up, I would just say, "That's something for you and your dad to do together." If it gets close, and he has not helped her get gifts, then I would let her earn a bit ( a little bit) to be able to get them something inexpensive herself and just think of it as her money, not yours, so that you don't get hung up on your money going to his house. The idea of that is no fun, but she would feel worse if she didn't have anything to give on a holiday.
My kids actually got their dad a $5.00 bike light one year when his car had broken down on the side of the highway and he left it there never to return. He was relegated to a bike as means of transportation for a while, and he was actually thrilled because he had been pulled over on the bike by a cop for riding without a light or reflectors at night. It really is the thought that counts!