Well first of all I would look into the lost property process for whoever is running that bus service. See if the shoes can be retrieved.
Second, are you asking "What should I do now?" or "How should I punish DC?" or "How can I make this less likely to happen again?"
Because the natural consequence is that now the shoes are not easily accessible and must either be chased up with lost property and gone to fetch etc or replaced at however much that costs, both involving the hassle of going to the shop/depot/etc and trying new shoes on etc, and the inconvenience of having no shoes or having to wear old shoes or borrowed shoes in the meantime.
If you're asking "how can I make DC aware of the effects of their actions" then getting them to do some/all of the above might be useful.
If you're asking "how to punish" then apparently the whole idea that consequences MUST be related/natural/logical etc is a load of rubbish and it makes no difference; involve them in the hassleful parts of replacing/retrieving the shoes if you want but it's not necessarily having a punitive effect (it might have a learning effect, which is great) but if it's punishment you want then it makes no difference if you ground them for a week vs say "Sorry you can't go to fun activity as we need to replace the shoes".
If you're asking "how to prevent/reduce chances of this happening again" then you probably want to do problem solving instead, this involves debriefing the situation to find out what went wrong, and building in safeguards against that scenario.
So for example, think up a way to attach the shoes to the bag so that when the bag is picked up, the shoes come with.
Or start a policy of checking back at the seat every time you get off the bus so as not to forget anything.
Of course you can do multiple of these things at once, it's not like an either or.