I can't tell what color your walls and ceiling are but I guess I would paint the ceiling white since it looks the same as the walls. As for the dark furniture, I would paint it the same color as your door and woodwork or white like you are planning to do. Ask your grands to make some colorful pictures which you could frame and hang up for color on the wall....or even a big bulletin board to put the pictures on and if they like TV, put a small TV on the wall, with a toy box under and some colorful floor cushions to sit on. Bringing in their toys will give you some color also. As far as lowering the desk and making a window seat....good idea but as the kids get older they might like a desk to build on, color on, study on and etc. I think movable floor pillows might be better. Looks like it will be a nice playroom.

Well, there's certainly nothing that says "play room" or "inviting place to sit"! It's just a mostly empty room with some office furniture in it. Think about what you'd like this room to be like, rather than what furniture you have. Low table and bins for craft stuff? a lot of cushions on the floor and posters on the walls? A big comfy couch and a bookcase? Colorful rug?

Here are some whimsical and inviting spaces that might give you an idea or two about how to cheer the place up, even if only a direction...

Hope this helps! (and that I've understood your goals correctly, heh.)

Paint the black furniture white or the same beige as the woodwork. If you don't care for the Venetian plaster look any more, choose a new color scheme that will still coordinate with the rest of the house.

A playroom is best with toys specifically for kids, not leftover clutter in old storage units. Make it look fresh and stock it with activities just for the kids. A reading sofa that will hold grandparent and kids is a better choice than individual chairs when the kids are small.