Dentists pls help: a groove/line on molar

[newbietoo]

My kid has a groove all the way to the gumline on the molar and the molar is probably not fully grown yet. I have a groove on my molar too when I check on myself. The problem is that the dentist said that the groove needs to be fill up to prevent cavity and has to be done within 1 month. I don't know how long the groove has been there but the last check up my kid had was 6 months ago? What's the urgency of getting the groove fill up if it has been there for a while and for me, I have my groove on my molar for at least 30 years now and had no cavity on that spot. I need to feel comfortable with the suggestion a dentist gave to us but I just don't understand this one. I feel that it's not really that necessary to fill the groove since mine has been with me for 30 years and didn't cause a cavity. I asked the dentist if the molar has fully grown eventhough I know that it hasn't... just to see if he might think it is better to wait to get it fill than to do it twice. Moreover, if my kid's molar is not fully grown yet, don't I have to pay again to get it fill up at a later time again. Dental cost is high under our insurance plan and we can't afford it unless it is really necessary. If my kid brushes teeth twice a day, wouldn't that prevent cavity from forming on the groove? Do we really need to spend money on this incidence? It will probably cause us about $150 under our dental plan. Please help reply asap. Thanks.