The best way to tell her to butt out without sounding mean or disrespect is to word it this way, "Mom, I know you only have mine and your grandchildren's best interest at heart but I'm an adult and things aren't done the way they were when you were bought up. Though I respect your opinion and guidance but this is my life and only I can live my life." Your mother is only concerned that the children are going to see something that they shouldn't see and that is typical grandmother reaction, she only wants the best for you and her grandchildren so don't get too upset with her. What you are going to have to do is keep your relationship just that your relationship and you're not obligated to divulge all information with your mother. Maybe next time pay a few extra dollars and get a double bed and put the two children in the bed together and then you and your boyfriend share a bed. You have to put your foot down now before she ruins your relationship with your boyfriend or you and your mother have a falling out.
Dating when you are a single parent can be difficult at times and with it brings many decisions as far as how much your children should and shouldn't see if you didn't have sexual intimacy with him then there really isn't anything to worry about, your mother is just worried how the children will see that morally. Everyone is different and have diverse opinions on this. Some people feel introducing a new partner right from the start is a good idea; while others feel they should wait months or even years before introducing their mate to their children. There is no ideal way of handling this with child; it is down to each situation and your personal judgment. If you feel this will not harm your relationship with your children then that is your choice as a mother. Maybe you should ask your mother does she really not trust you to do the right thing when it comes to your children, ask her if she felt she did a good job raising you and if her answer is yes then ask her to trust you enough to know the right things to do with your children. Make sure when you talk to her it's in a public place so that she is more apt to stay calmer and not start an argument. Tell her you would not do anything in front of the children. Tell her that she raised you well and now she has to trust that you apply what you were taught in your everyday life but that you deserve to be happy also and that her opinion is just that an opinion and that you have the final word because they are your children and you know what is best for them. If you say it in a way that isn't confrontational she will be more inclined to listen and it will also give her food for thought.