Is the Isa a cash Isa? Stocks and shares ISAs have much better rates of return (but also increased short term risk of course).
I'm no expert, but for my dc we invested the JISA in a low cost tracker type fund. We went for the Vanguard Lifestrategy, but there are plenty of others (an overwhelming amount of choice actually). I wanted medium risk and was leaving the money for over 10 years so both the 80% and 100% equity had an acceptable balance of risk/reward to me.
We looked around for the best value platform at the time, and went with Fidelity.
Vanguard now also offer their own platform, the fees are very low but have to invest a certain minimum a month (think it's about £200) which we don't.
So it is possible to have a Vanguard fund via the Vanguard platform, or invest in a Vanguard fund via another platform.
My advice is to look for both a fund and platform with low fees, as otherwise the fees really eat away at your rate of return especially once you take compounding into account!
Average return has been about 10% a year, which we are happy with. I am expecting that it may well decrease in value over the next year if the doom laden stock market predictions are correct, but this is fine as we have over 10 years to go.
We will probably start to move some of the money into a lower risk fund (more bonds, less equity) when dc approaches the age they will need it. (Which is the approach many pension funds take, I believe).
My issue with choosing a fund was that there was SO MUCH choice, I was almost paralysed with indecision! In the end I accepted that it would be impossible to choose the best fund, but with research I could choose a good fund. And a good fund is far better than leaving the money languishing with a crap rate of return.
When it comes to choosing a platform, I looked for low fees and FCA regulated.
But again I am not an expert, and so my advice may be wrong.