You can't really consider any arc filler, if you take into account of all the arcs that have taken place there has been some form of character development or expansion of lore. I think off the top of my head the the elf arc may have been seen as "filler" but it did develop a lot of things. It expands on Guts nature of being both sympathetic but still having that deep rage against anything related to godhand which would later develop into his own inner demon. The inner demon is further exposed when he's by himself at night attempting to fight the poison and goes into a berserk mode killing everything in sight kinda like how the Berserk armor is now. Though he does manage to gain his sanity before swinging at Jill.
His whole attitude during this arc seems to be an inner battle between having comrades and wanting to be by himself so that he can fight with no limits. It could be that or possibly when he tossed Puck aside he knew that Jill was going to follow so he wanted him to protect her. He also questions his own actions as if killing them is the only solution and slightly ponders if perhaps it would be possible to change them back into children. With the whole idea that upon death they change into children it builds up the Vatican's fear that Guts is a force that should be stopped at all cost. I think the arc also showed that regardless of ones use of the behelit it'll always have dark consequences regardless of how "inncocent' they want to be.
With this arc all the girl wanted to be was an elf like in the fairytales, using her parents as the catalyst to gain her powers. With her powers she was given the ability to change other children into elves however the blood lust from the behelit brings their true forms as oversized hornets/bees. With their bloodlust they ravage villages and burn down crops destroying the livelihood of the adults. While some may want to believe that Griffith might be doing good with his Behelit this arc supports the theory that the behelit brings nothing but evil.
Well honestly I just reread the arc and there's a lot of development from it, I just recall that that arc some consider a "filler" yet if you look at all the info give you would see there is a lot of information given. With that in mind though what arcs would people consider filler, I"m sure if you actually read the arc as a single entity you would notice content that expands on the story. When I think of filler I think of content that does nothing to expand the story but with the lost children we learned a lot about Guts inner battle, how far his insanity and rage drives him, the Vatican's continued chase after him, not to mention the characters Serpico and Farnese which later become part of his journey. But ya, I've prolly gone completely off topic lol