Hard to rate since the OVAs are very different from each other.
One is called "Hey! Butts!", is unexpectedly about sumo, but as indicated by its telling title goes the fanservice route. Bearable, but an easy skip.
Another is best summarized as "swimsuit tentacle rape", do with that what you will.
The last three are focussed on the summoned kids. These are genuinely awesome and what I would wholeheartedly recommend watching these OVAs for.