"Les gusta ir en bicicleta" or "Les gusta hacer ciclismo".
As you know, "I like" = "me gusta". The indirect pronoun "me" refers to "I". Therefore, when you want to say "they like" you have to choose the appropriate indirect pronoun. In this case, the indirect pronoun of "they" is "les".
This is a list of indirect pronouns:
I = me
You = te
S/he= le
We = nos
You both/all = os
They = les
I hope this is what you need.
