Shomor v’Zochor:- 2 sets of commandments, hence 2 Shabbos candles, period!! I don’t add Chanukah candles for kids, and likewise don’t add Shabbos candles for kids.
If one ever forgets to light {and not for the reasons that you “adorable” state that happened in your circumstances} and does have to light an extra candle, this only continues until the following Yom Kippur inclusive. Clearly after being forgiven by haShem on Yom Kippur for having missed to light that time, your slate is cleaned, and back to the regular 2 candles.