L.R.
Both of my daughters, 3 and 6, do the same thing and have for a few years. It is super healthy but I, like you, don't want them sitting on the sofa every afternoon doing it. I remind them that it is a private thing that needs to be done in the bathroom or bedroom when they are alone. It takes constant reminders for the 3 year old but the 6 year old stopped doing it in the main rooms of the house when she was 4 or 5. Try not to make a big deal out of it.