This is way too simple, but my children just loved mashing the veg. They would have gone through kilos of potatoes and carrots if we had them. We also tried with potatoes and spinach and they liked not only mashing but also, surprisingly, eating them.
You need :
25 gr butter
2 tablespoons double cream or creme fraiche
Boil the potatoes and carrots (together) in salty water. When they are very soft, drain the water, add the butter and cream and mash the whole lot. Drizzle the olive oil on top... and done.