Your kids have a few decent options.
Nutmeg is a nice digital option which has good visualisation and is good to track on a mobile. You need a minimum of £500 to get going as a minimum plus regular savings of £100 a month. The charges are about 1.15% all-in but they will build and manage all of the investments. Even if your kids don’t put their money there, I’d suggest they check it out and do the ‘risk profiling’ journey.
If those minimums are too high, then you can get going on an investment platform. Charles Stanley and Fidelity are the cheapest options for smaller amounts and you can start here with direct debits from £50 a month. If your kids do think money is boring, they won’t want to faff around initially with choosing and research all sorts of funds. So I suggest what we call a multi-asset fund. At this early stage, the most important things are just to get started and to keep costs low. So I would suggest the Vanguard LifeStrategy funds. The only choice they will need to make is how spicy they want this to be. They could have the most spicy 100% equity option or a much less spicy 40% equity version, for example. This will depend on their feelings about risk and the timeframe they are investing for. Given you mention ten years plus, and their ages, I would consider the more spicy versions.
If they buy a Vanguard LifeStrategy fund on Charles Stanley, for example, this will cost about 0.49%. That is a very cheap way to invest! The visualisation will not be as good as with Nutmeg.
Finally, I think financial markets are weird at the moment. No-one knows what Brexit will mean. Government bond yields are crazy low – less than 0.7% for a 10 year bond. Interest rates are at historic lows. The FTSE will have some volatile times ahead. They will need to prepare for their £100 to turn into (possibly) £80 and (hopefully) into £120 and then back down again – it’s the nature of the beast! But over 10 years we work on the historic belief that the market will outperform cash.