To Denmark is it not possible via DB, but to Brussel it should be possible. 

Thanks for the quick reply!

Not possible..hmm so I have to reserve via Interail?

Or via obb, sj..,..? I have all the apps on my phone. 

This Interail adventure is becoming so complicated.. supposed to give you freedom, flexibility..

NOT app-the site bahn.com-use ´seat only´-its mostly 4,50 (5,xx for 1st cl).

If you can only use fones: have to load DB-navigator, but if thats worth it for just 1 trip-up to you.

ICE trains do not sell out-there is NO controlled access, but when DB feels it gets very crowded, they post a message like ´quite full- REServ not possible anymore)-(as I always use /de/ it may be a little different in /en/). It sometimes indeed happens that trains have to be cleared from standees.

You can always, always reach BE via local trains-this may take a while longer. DK mostly too-change Flensburg to DSB starting from there.

Its also unclear your Q about cancel: it all depends on what railway/trip etc-those from DB for ´seat only´ can be exchanged-untill a certain time before its valid-in person at a counter of them.

For questions like this, please give details: route, date and departure time(s).

It may be the case that there are no reservations left.

For the mandatory reservations Hamburg - Copenhagen, you can book them online via DSB. (DB can book them online but only when they're optional, which is outside the summer. At the DB ticket office or by phone, it is always possible.)

Optional seat reservations done via DB cannot be cancelled.

Thanks both, so what I conclude:

ICE trains: only can be booked with db when reservation is not mandatory and these reservations can't be canceled.

I'm travelling with my child, BRU-HAMB 15 Aug and HAMB-COP 16 Aug. Can we risk not making a reservation for the first one? (First class Interail)

Second one seems to be necessary so I'll book through DSB. (Another app to install )

For DSB you don't need to install an app, the website works just fine :)

For Brussels - Hamburg 1st class on 15 August, reservations are still available for all connections starting with an ICE in Brussels.

For some connections however, Flixtrain appears where Interrail is not valid. You can avoid that by unselecting "Interregio and fast trains” in the app options. (Please note that the app will keep this setting until you reset the options.)