The urge to make games is strong in this family. I've just finished playing our own customised-for-a-five-year-old version of Mobile Frame Zero, although it was probably closer to Brick Battles than anything else.
Every time we finished the turn for a mech, a new rule appeared. Tough mechs. Fast mechs. Repair stations. Respawning. The list of rules just kept coming! Somehow they all meshed together well enough for us to play.
I'm glad I bought Brick Battles. Yes, I bought the little booklet! Totally worth it.