So many posts on this subject & I hope to chime in on more than one as time allows, but to start here, getting 2 1/2 yr. olds to pick up blueberries is not a battle I would pick.
I totally relate to much of what BBG & E&H have posted. I'm pretty no nonsense, & dh & I are definitely structured parents. And I agree once you engage, you can't let them win.
That said, at 2 1/2, I wouldn't try to get them to pick up the blueberries if they don't have naturally compliant personalities. Rather, at the 1st throw, the food goes bye-bye. I would in no uncertain terms say, "We do not throw food!" & remove the food.
I have never spanked my kids & it's not a tool in my tool belt. I will never say never, but it just hasn't been an issue thus far. Once engaged, though, I agree you can't give in. Kids seems to learn so much & immediately from prevailing in a battle of wills with parents. Sigh.
I would focus on minimizing the food issues by cutting them off at the pass should they commence rather than trying to push cleaning up. Now, at 4, ds does have to clean up if he makes a mess, but at 2 1/2, it wasn't worth what it would've taken out of us. And ds #1 is a compliant personality. We have challenge in correcting him without crushing him, but at 2 1/2, I still would've just ended the meal.
Here's the great thing about parenting, though. There is always tomorrow to change your course if you decide it's best. I've posted before on occasion that dh & I aren't always the quickest to spot an issue, but to our credit, we eventually realize, hmmm, this has become an issue & from there, we decide how to handle it.
What are your big issues? If food throwing is one, decide in advance how to tackle it. Bless those who think fast on their feet, but we do our best work with contemplation.
I would just cut off the meal. And I'm not above guilt/lecture. I would probably go with the "we don't throw food & look what you did to mommy's floor - not OK" version. And I mean it when I say it.
Hope that helps some. I acknowledge how much harder all of this is with twins & doing it solo, but I would not pick this particular battle. I would just cut it off at the pass. (And I do pick plenty of other battles. ;P ) Set yourself up to win the battles you do pick & minimize the damage for the others.
And while it depends on parenting style, for us, picking our battles is NOT wimping out at all. It's all about strategy. Bwahahaha, kids.