Fun with good graphics. I like the friend having to patch the hole everyday.
Personal taste but I like when upgrading help me to storm easily through the beginning to reach later stages quickly. The digger would do that. The cooler tough only help at the center of the earth and doesn't change the game experience. So when you are able to get there, you have to do multiple runs just to buy coolant to get nearer without change to the game play. That pretty much slow progression and suck the fun. Also I think the health should be in the form of a life bar instead of health units. This way, early obstacles could do less damage and later ones could do more.
Last thing, the first two versions of the radar are useless. Other game have found ways of making multilevel radar where even the first one was a good help.
I like the imaginaion of your game. Makes me thing of the other one with the boy who wanted to destroy the moon for fear of werewolf. Could have been fun to have a little more backstory. Like knowing why the little boy is so motivated to get on the other side.