It seems to me that lowering the monthly cost is precisely what they'd prefer. 8 years later both parents are probably back to working full-time, and with promotions etc they will have better economies.

Why would a young not so well off family prefer to pay 35 dollars a month for eight years, to paying say 20 for 20 years?

I'm really confused by this because it seems that it would benefit both your customers, and you, since you could charge more overall while imposing a smaller burden. Plus it would sound cheaper.