Regaining Control Of Children?

I have 3 kids all girls Cash Cahm and Payton. cash is 4 cahm is 3 and Payton is 7 months. I have always been strict with them but when i got in a serious relationship with my current boyfriend clint all my authority went out the window. They listen to him perfectly he says naptime and they go right to bed. They listen to me perfectly when he is around but when he leaves i am so suprised with how they behave mainly my three year old. When cash was three i had this problem with her. I am all about persistence but its hard to leave my baby to deal with an ongoing thing that seems to have no end. I even try ignoring them sometimes i just dont know which way to go. Me and my boyfriend are on thin ice and i sometimes feel like i stay with him because my kids listen to him. I want to regain controll weather we split up or not i am their mother and i want to fixnthis problem while they are still young. Ive tried reading but seems like it doesnt help so i thought i would post my specific situation on here and see what response i get. Nothing to lose right. Anyways looking for advice