This might sound funny, but has worked wonderful with my daughter in the past (as well as several friends). Try making up a story about bedtime that you can tell them during the day when they are happy. Tell them it at a typical story time or maybe while talking/visiting during a meal. I would say something like: "Would you like to hear a story about 2 special little children? Well, there are 2 special children who have such fun everyday. They love to ride their bikes, paint, read books, and play with their mommy and daddy (adjust to your situation/likes). Everyday they play and eat and play and eat, and at night time they go to sleep. When it is time for bed, the children love to take a bubble bath, read a book with their mommy, and then they happily go to sleep in their bed. They are so happy and pleasent that their mommy loves to be around them and do special things with them. This is what they do and they all lived happily ever after." Wow - wasn't that a good story! Now, do you think you might like to do that? Well, at bedtime that is what we will do. We will take a bubble bath, read a book, and then you will go to sleep and be happy. Let's try that tonight!
Give it a shot! Be enthusiastic and positive. Sometimes kiddos just need a little encouragement, and to know what is going to happen. You are setting them up for success. If they cry, just remind them about what you talked about earlier, and that the children are to go to bed happy. Then, in the morning, do something special, make their favorite breakfast, and you can all live happily ever after!