Hijikata has seen a lot of horrendous things in memories from Eren's world, so I have half a mind to give him a break and let him see the little trio being happy! Or we can swing the other way, and this event can be how he finds out that Eren is going to end the world back home (though I'm basing this on assumptions; I haven't watched the video yet because I'm close to getting there in my AoT watch this week)... decisions!!

In any case we can handwave the memory-watching and cut to the confrontation afterwards (which will of course only happen in the worse-case scenario, otherwise Hijikata will simply be glad that Eren at least had a happy childhood up to a point). Whichever you prefer!