Is there a single stock/etf that's more or less guaranteed to at least keep pace with inflation over the next ~20 years?

I've just set up a junior ISA for my daughter and don't really want the burden/responsibility of maintaining a balanced portfolio. I'll be putting in a set amount each week/month and just want to pile it into a single instrument. I was thinking maybe an index tracker or something. I'm not chasing gains or anything, I'd just like her to have the same value of the money that's gone in (adjusted for inflation).

We're not talking much here, so getting an accountant/fund manager involved is probably overkill.

Any ideas?