I bought Magic Hill over 7 years ago in order to play it with my children. My sons, (finally!) ages 7 and 5, are able to play Magic Hill with much less frustration with this simple variant.
Instead of 'seeding' only the final level with a secret passage, I seed each level since there are 4 secret passage chits, that makes 1 for each level. So I take out all 4 passages, mix 1 in with 7 other action chits for the first level, then 1 with 4 for the 2nd level, then 1 with 3 on the 3rd, then finally the normal 1 secret passage with 2 other action chits on the final level.
Again, these allows them to ease into to using the piggy back method for rising levels as is sometimes required in the regular game.
Let me know if you try this and how it works out for your family.