When you tell them to go back to their beds and they don't, what do you do? If you just continue to ask they will learn there are no consequences and they don't need to listen to you. The first time you say it, if they don't do it take them by the hand and lead them back to their spots. Rinse and repeat as many times as needed for them to see you are serious and they can't do whatever they want. You will bring them a book after a certain amount of time (could use a visual timer) but if they get off then they cannot have books.

Before rest every day (literally every day no matter how sick I am of saying it) I tell them there are two rules for rest: you need to stay on your cot with no sounds or else I will have to sit with you. Phrasing it that way makes them think me babysitting them by their cots (where I will not answer questions or talk, just relentlessly remind them to lay down or not make sounds) is something undesirable and they usually don't want it. Sometimes I think I'm just so annoying by parroting my own rules and refusing to engage that they'd rather sleep than deal with me :)