I want that same innocence my first had to be preserved for my two youngest. I'm not blaming him, but it does make it harder to keep those influences that are unavoidable for a middleschooler away from pre-K and grade schoolers.
I understand that urge, but speaking as the oldest of 5, unless you wait to have another kid until your first is off in college, they ARE going to influence the youngers.
I have a close in age full brother, then our half brothers are 13 and 15 years younger than me. Then they have a full sister 10 years younger than the older one. The brothers weren't all that influenced by me b/c I didn't live with them, but the sister only ate breastmilk (mom and stepmom and the rest of the family are extended/biological nursers), candy, and rice, b/c bmilk is what was offered by mom, and candy and rice were the ONLY things the boys ate! The boys watched her quite a bit, since their mom was in nursing school and my dad worked nights, and candy is the only thing the boys gave her. Ever.
She also grew up with a really big vocabulary, and she's smarter than all but my full brother, b/c she learned the GOOD stuff, too, from her siblings both full and half.
There's good and bad with older sibs, and I understand WANTING to be able to keep it all away (until my son was 3, he really believed that the cable company simply turned off the TV after Little Bear, b/c it was time for a nap) but unless you want to take everything away from the older, it's going to be an uphill battle.
I think it's super-cute that the 3 year old TRIES and apologizes. He's getting it!
As for those substitutes like OMPoodle and OMGoodness...friend of mine has a mom who charges for swears, though her kids are all in their 30s and 40s. And those substitutes COUNT. Gosh, goodness, even poodle, if said in that context are all just substitutes for what they are meaning, and you gotta put that quarter in the jar if you say them around their mom.