If she is willing, take £4k out as cash and pay that in to a Cash Lifetime ISA - use it to build up a first property deposit. Cash Lifetime ISA because buying a property is likely within the next 5 years. If the time period will be longer then could consider using a Stocks & Shares Lifetime ISA.
I don't know if a transfer counts for getting the Lifetime ISA bonus, which is why taking it out as cash may be necessary. I would check with the provider of the Lifetime ISA as they may have a way £4k of the CTF can be transferred to a Lifetime ISA and get the bonus with the remainder transferred to a ISA.
Withdrawing from the CTF at age 18 I think is Tax Free, so there should be no issue with taking some out as cash if that is necessary, rather than doing a transfer.
The other £5.5k could be put in to a S&S ISA. This is then accessible whenever they want but with the intention of it being there long term.
They could also split with putting some in to a Pension or SIPP. This option would depend on if they have earned income. Without any earned income the max they can put in to Pension/SIPP is £2280 per tax year. If they have earned income, then they can put in to pension up to their earned income or £40,000 in the tax year, whichever is the smaller.
I'm thinking of putting some money into the fund before the end of this tax year and maybe some at the start of the new tax year to max out how much she can have in the account as I think if she leaves the money in the matured product she cannot add to it. Does this make sense?
Not sure what you mean. She does not get access until she is 18 in the Summer, so I can't see how you can do anything with it before 6th April 2021. Certainly once age 18 it should be moved as quickly as possible to another wrapper... be that Lifetime ISA, ISA or Pension/SIPP.
Something else to consider:
£4k to Lifetime ISA in Summer 2021 - remainder as cash, could put it in Premium Bonds.
£4k to Lifetime ISA on 6th April 2022
At this point Premium Bonds would have £1500, which is a good starter emergency fund, for urgent car repairs, visiting a sick relative costs etc.
Lifetime ISA would have had £8k paid in and got £2k of bonus, so come about June 2022 there would be £10k in that.
What they do with the money when they are 18 may depend on what they are doing when they are 18. If they are going to further education they may have some costs for things to pay towards for that. They may have driving lessons to pay for, a car to buy, car to maintain. So I would encourage them to invest most of it and spend some of it on things that are Needed to help them through the next few years.