It will be life changing. In our last house we used a 10' x 10' conservatory to stash all the children's toys (they are 7 and 4) mainly in a Trofast Storage system from Ikea.
We moved and gained a bedroom but that is now DH's office so we have no playroom anymore. The double garage is going to have 4m of storage along one wall opposite where a sofa bed will be going.
This is hopefully going to mean that the lounge is more of a grown up space.
All their toys, books and games can be stashed in the new room, I am literally taking a break from painting it as it was only finished yesterday 
Make sure that any company tells you what they are going to insulate the room with. I was lucky that I am a sahm so was here for the whole thing, plus as I already knew the builder I could ask lots of questions.
We had jablite (like a polystyrene) floor with a chipboard top then a foil backed underlay then laminate. The walls were insulated with a foil thing on the single skin of brick walls, then timber framed which was stuffed with loft insulation stuff.
Just think about where things will go like sofas, tables, what do you want to use the room for? Make sure there is enough seating for everyone, sofas or beanbags.
By the sounds of it with 4 DC utilising the space will work well. Although we have retained some storage at the back we are putting a shed in the garden too.
Get loads of quotes and go and see some of the work they have done. As you know some people's standards differ vastly.
Good Luck!