I know what doch means, I don't know how to translate it. ;)
The thing is that "Wir freuen uns" is always translated as "we are happy" or "we rejoice" and with this "doch" they add the "let" thing, "Let's be happy", "Let us rejoice".
It's pretty confusing. Especially for somebody who has to learn the meaning of "sich freuen" here! ;)
OK. I tried "Anyway, let's be happy", but it was not accepted. Some problem in the sentence?
I am thinking/feeling that the "doch" here expresses something like "we have good reason to be happy anyway" - even if to be happy is the reason itself. "Doch" suggests to me that there is something else which can/could be a cause of not being happy.
Duolingo is teaching you how to enjoy yourself? That's good to hear ;)
Sorry, I wouldn't know how to translate it either apart from the description I gave. It could be translated as "but" in a sentence like "Keine Sorgen, freuen wir uns doch", but I feel that it doesn't necessarily have to translate like that.
"Freuen wir uns" is clear, it is the imperative of "freuen". "Let us rejoice" would translate this sufficiently. The "doch" is used in a case where you get or experience something negative, are not satisfied with it, but can be regardless:
Ex. 1: You get a car from a friend for free but you need to fix it.
A: is complaining
B: Freuen wir uns doch (we have a car now).
Ex. 2: There was an earthquake and your house is damaged.
A: All my stuff is broken.
B: Freuen wir uns doch (we are still alive).
Something like "Anyway, let's be happy" as MaheshMarg suggested above or "Regardless, let's be happy about it" expresses this quite well.
Intonation: The intonation is wrong here. The stress must be on the "freuen" (and not the "doch") as this is what the sentence expresses: "Regardless of what happened, let's be happy."