They are not tied in. They can leave with standard notice like any employee, and the qualifications they have earned are theirs to keep for good.
What we do ask is that training we pay for ourselves is repaid if someone leaves soon after we have funded it. That covers extras such as driving lessons and additional certification we buy on top of the government-funded qualification. It does not cover the qualification itself, which is funded and is never repayable.
The amount reduces the longer they stay and falls away completely after twelve months. It is set out in a short written training agreement that you and your child both read and sign before they start, so it is never a surprise later. Any repayment is capped so that it can never reduce their pay below the National Minimum Wage.
We would still much rather someone worked out in month three that it is not for them than spent two unhappy years here.