Depends on what your daughter want out of the hobby. If she wish to see them multiply and watch the babies grow ( very likely) then the driftwood and some dense plants would help. If its just keeping male guppies for show, then its okay to have a bare tank. Fries get eaten up easily in bare tanks as they are unable to hide. As for the filter, covering tthe inlet with a sponge should do the job. Alternatively, u may choose to use multiple ( because of the size of your tank) spong filters.
Just my $0.02
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Aquarist.