In reply to James Malloch:
To travel into Germany as a non German citizen you need a valid reason.
According to the police website valid reasons are amongst others:
Familiäre Gründe
Besuch des Ehegatten, Lebenspartners, Lebensgefährten
Besuch von Verwandten; insbesondere Kinder, Enkel, Eltern, Großeltern, Geschwister, Tanten, Onkel, Cousins und Cousinen
Sorgerechts- und Umgangsangelegenheiten zur Betreuung von minderjährigen Kindern, dies beinhaltet auch das gegenseitige Besuchen oder Abholen
Teilnahme an wichtigen familiären Anlässen, wie Hochzeiten, Begräbnisse, religiöse Feiern, auch für andere Personen als Familienangehörige
https://www.bundespolizei.de/Web/DE/04Aktuelles/01Meldungen/2020/03/200317_...
So visiting a father is OK.
The quarantines are over since last weekend for EU citizens (inc UK):
"The federal and state governments have agreed that all persons entering Germany from third countries are required to remain in self-quarantine for two weeks. Third countries are countries outside the European Union (EU). Persons travelling from Iceland, Liechtenstein, Norway, Switzerland and the United Kingdom of Great Britain and Northern Ireland are exempt from this requirement, as are persons travelling from EU countries."
and
"Persons travelling from the European Union, Iceland, Liechtenstein, Norway, Switzerland or the UK can be ordered into quarantine if over the last seven days the number of new infections in the respective country amounted to more than 50 per 100,000 inhabitants. Please consult the website of the Robert-Koch-Institut to find out which European countries are affected."
https://www.bmi.bund.de/SharedDocs/faqs/EN/topics/civil-protection/coronavi...
So should be fine as well, as long as the situation does not deteriate massively in the UK.