Set a timer and make it a game. "I bet you can't get dressed for bed before this timer dings!" Or say something like "who can put their shoes on faster? Susie, Sally, or Mommy?" Say things like "Oh I don't know, I think mommy is faster." You can bet they'll want to prove you wrong!
I know you don't want to take away the reading, but I can honestly bet that if you did it just once they would miss it and it would be incentive for them to 'hurry it up'. You don't "take away" the reading but you tell them this: "Reading and cuddling starts at 7:00 and ends at 7:20. We are still going to bed at 7:20. If you aren't ready by 7:00, you are choosing to be slow in getting ready to bed and cutting into your reading time."
In the book "Healthy Sleep Habits, Happy Child" by Dr. Marc Weissbluth, he uses a case study of one of his patients and parents who had a similar issue. The child would play and play and then be pokey joe when it came time to bed. One of the kid's favorite things to do was to cuddle and read with mommy at bedtime and they would literally sit and read for nearly an hour. Of course, the 'pokey-ness' pushed the reading time back farther and farther, resulting in the child going to bed later and later. The child had absolutely no incentive to get going at bedtime; she knew could poke around as long as she wanted because she knew she was getting all the reading time. Dr. Weissbluth suggested setting a timer for a period of time before bedtime, and when that timer went off it would be time to go to bed, no exceptions. He even suggested that the child set the timer. Think of how empowering this is for a young child, especially ones who want to be in control of everything! The child will know they have a set amount of time before bed, they set the timer, they choose the activities they will do before bed, so if they choose to take their sweet time then they choose to forgo the cuddling/reading time. If your kids aren't into the reading time and would rather waste their 'timer time', then you can always adjust it so that they have to be ready for bed by the timer goes off OR the next night, the timer gets set earlier, thereby building in an extra 30 minutes for the reading (the 30 minutes gets cut out of their play time...as soon as that bell rings it is time to get dressed and ready for bed).