If you don't have lounge access, I think it's worth going through immigration - even if only to stroll around the landside restaurants and stores. The airport observation deck appears to have a play area with a slide that might give your five-year old a chance to let off steam:
https://www.kansai-airport.or.jp/service/other/09.html

If you cross the bridge to Rinku Town, you could take a look around the malls there or watch the sunset from the beach.