Not sure if it's something my 4yo did but he got the level 1-1 acheviment and now the game won't open.

If any account tries to open the Alien Hominid HD game it opens to the initial screen with the alien holding the little console computer. It's blank. I hit the little circle in the center of my controler and for a brief second I see the words. Then I jump back in and nothing.

Few things my kid was doing was;
1 - logging out while in the middle of the game.
2 - Switching to different players on the controler in the middle of a game. 
3 - Reformatted the hard drive.

I've tried to delete the game and download again. Just down load and still nothing. Please help. I want to play this game but can't get into.

Might try clearing the cache.

Go to the System Blade, and select Memory.
Highlight the HD symbol and press Y.
Press X, X, Left Bumper, Right Bumper, X, X.
Confirm.

I'm not sure what those instructions are for.  There doesn't appear to be any way to explicitly do what you suggest from the System Settings -> Memory -> Games -> Alien Hominid screen.

I also had this problem as well, but I noticed someone else talking about keeping their Rock Band controllers unplugged because of save game issues.  I tried this, and it seemed to work.

I agree with that controller thing. If there are Rock Band or Guitar Hero controllers plugged in then they should be unplugged. I noticed that when I played NHL 07 or NHL 08 and the Guitar Hero guitar was plugged in the game went to online mode all the time because it seems that guitar controller has "left stick" going right all the time. Unplugged it and, yay, could play single player normally again.

And about that cache clearing. I'm sorry, I  told you the old way cause I just noticed this a while ago myself that after some update there's now a button to clear the cache. You have gone too far if you are in games and alien hominid saves though. Just highlight the HD and press the X button (don't press A), or whatever it says in the bottom of the screen, to go to device options. There is now a "Clear Cache" button so you don't need that button combination.

Be warned clearing the cache removes all the game updates from the HD. But on a second thought. I tried to "cheat" on Alien Hominid HD to get that perfect fight achievement with a second controller but clearing the cache didn't delete the update from that game (had to get that achievement the hard way). If deleting and re-downloading doesn't work you could try clearing the cache but I'm not sure if it helps.