気をつけて帰ってねって英語でなんて言うの？

It really depends on what mode of transport the person is using to get back home. The three possible modes of transport the person may use are , walking, public transport, or driving his own car. If he/she is walking back home, you may advise him/her to 'take care'. If he/she is using public transport, you may advise him/her to 'stay safe'. If he/she is driving his/her own car, you may advise him/her to 'drive safely'. So, you may say: Stay safe on your way back home. or Take care as you walk back home. or Drive safely.

To wish someone a safe journey is to say that you want them to reach their destination safely.

If someone is traveling back somewhere or going somewhere in particular, you can tell them to "Have a safe trip." If they are driving you can say to them to "Drive safe."

Well, if your friend is about to walk through a war zone, it may be better to make alternative arrangements or even postpone the journey if you can predict it is a dangerous one. However, in a developed country, and if your friend is not passing through dangerous territory, the first example sentence is appropriate. The second sentence is a joke, being a reference to the children's story, 'Little Red Riding Hood.'

The first example is a direct way to tell someone, especially someone that you care about, to take care of themselves. The second sentence is something that we frequently do, which is that we ask that person who is leaving to send us a message or call when they get home so we know that they got home safe. This isn't quite as direct but your listener will understand that you want them to be safe.