House is 4 years old. Just moved in 9 months ago- no problems to this point. Last night dining room/kitchen (downstairs had) water running along a couple of seams with a small bubble.

Shower connects to tub. Walk in shower with tile on two sides and glass on other two. Very small tiled sitting area at back of shower (of note, there is lack of caulk/sealant where vertical and horizontal meet and there is a small opening in bottom right corner where sitting area meets bottom of shower- these openings appear to have always been there and not something that the caulk/sealant has eroded).

I opened up drain cover- there is standing water in pipe below and appears to be good seal from rubber gasket at top.

One last note- last night (when the leak happened) kids were in shower and on fell pretty hard playing around. Not sure if that had anything to do with it...

Not sure what the next steps are to trouble shoot leak or next steps to fix downstairs ceiling. Any help would be great.

I think your problem's in the tile where the shower stream hits it. But let's check the drain seal just to be sure, Remove the strained and stuff rags down there to close off the drain, Now fill the base with a inch or so of water and let u]it set. Keep a eye on the kitchen ceiling for any moisture leaking through. If non in a few hours then you should call in a tile setter to caulk and grout your tiles. Good luck, Tom