I think it depends. If the kitchen and bathroom were still serviceable, it is usual to wait for a change if tenant to do that kind of work. You do it between tenants when the property is empty.
If they were still serviceable but the tenant fancied new and just asked and you agreed, then I would probably agree on the basis that as it’s to suit them but at your expense, they just put up with the inconvenience.
If however, they weren’t serviceable and needed doing right now, then I would be looking to compensate them.
Actually it can be easier to offer a specific sum of money rather than a percentage off rent or for the period the works take. That way, the amount you pay isn’t so open-ended.
I had tenants in a property when it was being decorated externally. It didn’t really impact the tenants much as nothing was internal, but there was scaffolding and 2 weeks when the builders had to put a skip on the drive instead of their car. I offered several hundred pounds for the point when the work was complete. This was easier all round as the rent is paid via agents and it would have been complicated to adjust. The tenants were pleased to be having the decoration done and really pleased to be offered a cheque for several hundred. Receiving it in completion of work seemed to work well because it helped them tolerate the minor inconvenience and they knew they wouldn’t receive it until the work was done.
Internal work is much more disruptive, especially bathrooms, with access to the loo etc. It’s why I would generally avoid doing such work if a complete refit of a room until a change of tenant. When I’ve had very long term tenants, I have sometimes felt it was right to offer some upgrading as they renewed yet again...but it has been their choice about whether to accept the offer or not and as existing tenants they have usually understood that there will be some disruption, in the same way there is when work is done in a house you own and live in and any compensation payments would be quite small. This has been agreeable to all....tenants have sometimes declined the offer, but usually are pleased and surprised to be offered a kitchen upgrade or whatever.