I don't like to involve money with friends and family because I believe those two are bad combo and it can ruin the relationship over some silly things.
Well I agree with that that is why I just do that for some other activity together in exchange, like go to have a meal together, at the end when I get out with my friends usually we have fun and we don't matter if today one of us pay, we know that the other is going to pay.
In any case there is no rush to ask the payback right if you see that they aren't going to make it, then is better not to ask, at the end if you already fulfill the challenge it is still a win for you.
If that's the case it's fine but we never know there will be always the same level of understanding, things started small might escalate to big ones which is why avoiding it completely is better than leaving the room for it to happen.
This just remind me of my college days, where we usually do such things like involving small money over betting silly events and I am not sure it always ended very well which is why I realised later not mixing these can help us a lot.