I ned advice on father's day advice for my daughter's father and siblings??? PLEASE!?

OK, i have a 4 month old daughter but her father has two older children. They do not see their mother at all, but me and him are not together. I am not their step mom. Ever since my daughter was born, I have been getting closer to his children and I am falling for them more and more each day. His oldest daughter who is 11, is constantly texting me, and last weekend she asked me if I would help her get him a father's day gift. I accepted, and she wants to take him to mummy exhibit on Sunday. I am going to get them tickets to take him, but I am not sure if I should buy myself a ticket also. Its not like me and him are enemies, we go out to dinner or a movie every so often and we get along pretty well. Last weekend, we had a sleepover at his house, which consisted of his two oldest kids, my daughter, and me and him..... weird? Yet, we are not together. I am just not sure if it is appropriate for me to buy myself a ticket && spend that day with them. On top of that, the tickets are going to run me about 85 dollars for a half hour show!!!! So I am just like... ehhh.. but please.. I need advice. Also note, that his kids are constantly telling me how happy I make them and that they love me. I am confused as to what I should do!