Help with co-parenting, I feel like I'm doing it wrong.

Over the weekend, my boyfriend's 5 year old daughter came over (as per usual, pick up Friday, take back to mom's following Saturday).This past weekend we were all in the bathroom brushing teeth for bed (myself, my 6 year old daughter, and the 5 year old). She mentions me doing her nails, I notice she has gel polish on. I ask her if she just got them done, she said her mom did them. So, I didn't think anything of it, proceeded to remove it all and have her pick out what she wants. It was late and so I told her I'd do them tomorrow and got them in bed.Saturday came, when I woke up I got the girls ready, fed and took them to the park and for ice cream, after. By the time we did the park and other stuff we got home (approx 5:45 pm) and we didn't have time to do anything else but her dad had to get her bathed and ready to go to her mom's.Well, clearly I didn't have the time to do her nails.Now, it's Sunday morning and mom is texting me and DD's dad asking if I was the one who removed the polish, according to DD I was. Obviously, I told her the situation and why I didn't get to finish painting them, then even offered $25.00 to compensate for the incident.She proceeds to say no, it's fine, and she was just asking and says how frustrated she is because she spent 'good money' on the manicure and as a party approaching next Saturday and she wanted her nails done for the party. I apologize for a second time, even more sincere and insisted on sending the money, I even expressed how I understood where she was coming from and expressed how I was sorry to upset her.She basically said it wasn't a big deal. She was just wondering.Ultimately, I don't want another incident like this happening again because I simply don't want any problems; how can something like this be avoided? Is there anything else I should do to smooth things over?(I did send the $25.00 regardless, so she definitely got that) via /r/Parenting http://ift.tt/2lUqsOm