Return Flights and UK Standard Visitor Visa (for a minor)

but we're not sure exactly how long we'll be in the UK,

The main issue you (actually your son) face is what if you decide to stay for good in Northern Ireland. Surely that means he will stay with you and you will apply to change his status to indefinite leave to remain and/or subsequently citizenship. That is the question you will need to implicitly or better still explicitly address. They would want to be convinced you are not attempting to circumvent the standard process for bringing an adopted child to settle in the UK.

You need to have clear and convincing evidence that you (and by extension him) intend to return to Bolivia and that this visit is of short duration (less than six months) otherwise the route you should actually be using is the one for bringing an adopted child into the UK permanently which is more appropriately suited for our sister site, expatriates.

Proceeding with the assumption that your intent is for a short visit, and not to stay permanently we attempt to address your specific questions:

If I have my bank statement showing that I can afford the return ticket, will that be okay?

You are a British Citizen returning to the UK with your legally adopted and recognized son. You personally do not need to show anything, you can return with zero money in your pocket and a one way ticket. In reality you are only providing these details to cover your adopted son and it is better to be safe than sorry. Refer to the excellent answer to this question: Should I submit bank statements when applying for a UK Visa? What do they say about me?

And how much will I need to show I can afford?

The rules do not ask for a specific amount. They evaluate the complete financial position of the person. Yours is a unique case. A few hundred pounds in my estimation will be more than fine because once again, it is only for covering your son who is the visitor.

Also, we'll live with my parents there, and I'll be working during the 6 months (which I legally can) - will they ask questions if there isn't a whole lot on top of the money for the return tickets?

It is normal that family (your parents/his grandparents) provide some housing and room and board to family (your son). It would be good to have documented evidence that they (grandparents) are willing and able to house their grandson. Refer to my answer to the previous question.

CONCLUSION

My advice is to firm up your decision at this point before you apply, that is on the application be specific with how much time you intend to spend (you can always change your mind after entering the UK). To support that, you can purchase either an open return ticket (quite expensive), or even preferably a return ticket for say 3 months which has the option of paying a change fee to extend for up to one year. After you receive the Entry Clearance, carry the same documents you used to apply along with you on your trip.