Hi Adrienne,
Although grammatically correct 'on y arrivera bientôt ' sounds stilted in French and a mother wouldn't say this naturally when speaking to her children.
Because you want to convey that we are nearly there, the present tense is better.
and you could say 'on arrive bientôt'.
Whilst in a car journey young children would ask their parents in English -
'Are we there yet?' or 'Have we got there yet?'
which in French would be -
On est bientôt arrivés?
So Mum says - We'll be there soon. -
On est bientôt arrivés.
Hope his helps!