Go to SETTINGS->PROPERTIES->UPSELL ITEMS and add the tax as an "Upsell Item" with the settings
Type = obligatory % tax
Description = City Tax (if multiple languages are activated enter the description in all languages)
Children|Children under 10
Child|Child under 10
Then set your tax as "obligatory" "per day" or "per booking" "per adult".
1) add breakfast as an "Upsell Item" with "Vat %" = 19
2) add city tax as an obligarory % tax "Upsell Item"
To show VAT change your confirmations (SETTINGS->PROPERTIES->CONFIRMATION MESSAGES) and invoice template (SETTINGS->GUEST MANAGEMENT->INVOICING) to use the template variable [INVOICETABLEVAT].