I think the parent is surprised/upset at being asked to pay tax on a capital gain and to some degree I sympathize. It seems somehow unfair to tax what can feel like an existing possession. However, as I understand it a big reason for having capital gains tax is that without it rich people find ingenious ways to turn what is really income into capital gains and hence avoid taxes altogether. For this reason there are usually some thresholds and exceptions to allow some amount of capital gain to be tax free, and/or to tier the tax rate on gains.