Frosting Melting

[jeremyj]

I made a Thomas the Train birthday cake for my daughter and it was fine until I moved it to Chuck E Cheese, then all of a sudden the frosting started to melt.

The frosting was homemade

1/2 cup butter
1/2 cup shortening
2 lbs powdered sugar
1/2 cup milk
1 tsp vanilla

Any ideas on how to prevent the melting but keep the yummy creaminess..

Thanks.