Any cash savings over €20k will impact their JA entitlement. €100k in cash is assessed as the equivalent of weekly income of €270, see
here. This would wipe out the child's JA entitlement entirely.
My advice would be for the parent to hold on to the cash until the point at which the child actually plans to need it. The €100k gift will also impact lifetime CAT tax-free threshold currently €335k.
Some kind of trust may also be an option, other posters may be able to advise.