They are right to not want general messing about with dry ice, so that's a plus! You could instead use a mixture of baking soda and vinegar or put a cup of boiling water inside the cake to create steam. For each case, cut a round hole in the cake's middle to fit the cup, then add the soda/vinegar or hot water inside the cup and place the cup into the center of the cake just at the moment it is needed to be placed out on display. Use raspberry or strawberry jam for the lava running down the sides.

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