My family celebrated a belated Christmas earlier in January. My grandpa came in. He gave each grandchild, great grandchild and any spouses of grandchildren, $2,500. My husband and I received a single check for $12,500. (5 of us in our family).
His reasoning was that he and my grandma had very little money and her aunt sent them $25 when they were newly-married to buy a blanket. They felt "rich." And they spent $2 out on the town for dinner and a movie. Grandpa's idea was the same in giving this gift. In the letter that accompanied the check, he did not specify what we should or shouldn't do with the gift. He also said that they never did buy that blanket.
Separately, my mom pulled me aside and said that she knew he was going to do this. Then she said that she thought he was going to write us 5 separate checks. She said that his intent is that each of us get exactly $2,500 and make each check out to each separate recipient.
DH and I have our 3 kids' savings accounts and they each have 529s, which are well-funded. We have a small personal loan to DH's parents and we are saving for a used Toyota Sienna.
With DH's job and his future guaranteed income potential there, as well as income we will get from his family's farm, DH wants to take the $12,500 fully and put it toward the loan and car savings. I feel a bit torn. His reasoning is that our kids will always have what they need and we are being diligent about saving for them. He also sees value in not dumping a ton of money on kids so they learn the value of it and learn to work hard. I do agree with him. But, I have a twinge of guilt not putting each child's supposed $2,500 into their accounts. He said we could still do like $500-$1000 for each kid into their savings. So it wouldn't be like they were short-changed.
Getting rid of this personal loan and getting the minivan will greatly help us and them as once they are done, we can save more for us and them.
So, what would you do despite the fact that my Grandpa did not lay out "terms and conditions" for the gift? The only reason this is a question is due to what my mom told me. It wasn't that she was dictating to us, she was just giving us an FYI point.
Re: Totally unexpected...what to do...
I would probably put each childs' $2,500 into their 529's, and use you and your H's $5k to pay off the personal loan and put aside for the purchase of the van.
TTC since 1/13 DX:PCOS 5/13 (long, anovulatory cycles)

Clomid 50mg 9/13 = BFP! EDD 6/7/14 M/C 5w6d Found 11/4/13
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Chemical Pregnancy 03/14
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Riley Elaine born 2/16/15
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$1K for other items for the kids (summer camp, piano lessons, sports etc.)
Rest on goals for you and DH.