My DC are bilingual, DD1 is 5.5, DS is nearly 4. We all speak two (of the same) languages, but DH speaks a third fluently as he was brought up and educated in that country. Hope it makes sense so far... So far DH has pretty much refused to use that language with the DC, claiming that it doesn't come naturally to him after such a long time of not actively using it. He did, however, have a few attempts in the past, but nothing has ever come off it. We have just had a new baby and I personally see it as our last chance to introduce a new language so our three DC will hopefully grow up to be trilingual. Or is it too late now for our eldest two? Also, my DH and I are unsure as to how to begin and how it should be done in general. Gradually, a couple of hours a day (sort of like a language lesson) or dive right into it and never speak another word in the languages that we all use and understand? Those of you who had done something similar, how did you go about introducing a new language?
Disclaimer: DS is 3 months old but the theory is sound!
We will be (sort of are already) doing this. We follow OPOL, DH and I speak English as we're currently in a francophone country and DH, via a toy, also speaks Dutch. Everything to do with the toy (Miffy the rabbit) speaks Dutch and we have a range of accessories to encourage this later, like Miffy plates so he learns about food, books, DVDs etc. For once I'm grateful for the evil mass marketing of fictional characters.
If we moved to an anglophone country DH and I would make a concerted effort to speak French together but keep OPOL and Miffy going, otherwise I think we would risk not having enough French exposure and then Dutch would be dropped to preserve DH's mother tongue.
As to the question about older children, yes I believe they would pick it up but probably not completely fluently and you'd need to supplement your DH's possibly rusty/dated input with DVDs, books and ideally other speakers.
The biggest stumbling block I forsee is that your DH seems unwilling to do it. Is his language also the minority/non-community language?