Well, Henry and Gaius can be childish enough that the age difference is less apparent, but you've got them paired (oddly Sumia/Henry is the one pairing I haven't done for Sumia) already. I think that Vaike is probably the closest in age (and maturity level) to Lissa of the remaining ones. Besides, I think it's funnier than Kellam/Lissa or Stahl/Lissa.