Help! Bf moving in and my kids not happy about it!

Ok so my ex and I are giving a chance after 7 years. Feelings never left even with living with other people. I'm moving into a new house and my son who is 17 and daughter 11 barely found out that me and my ex are talking again. I told them we are planning on moving in together. They are mad at the fact that my ex and his 6 year old son will be moving in. It's not like if he's a stranger to him. He use to live with us when we were together and my kids loved him at the time. Idk why it is a problem now? What should I do? I really want to be with my ex it's our chance to make a life together permanently, but my kids are making the situation hard. Should he move in either way and hope that one day they'll like him?  Was it hard for any of you when children are involved ? I would like to hear if anyone had to go through this and some advice please.