No one can really answer this for you. It is a matter of personal preference and the details of your situation. There are some really smart people on here, when placed in your exact situation, would do completely different things. Personal finance is overall, personal.
If it was me, I'd never borrow money in retirement. If I had the cash, I'd use it to help fund the purchase. If I didn't, I simply wouldn't. For me wealth retention (in your case) is surprisingly more about behavior than math (even though I am a math guy). You are simply creating a great deal of risk at a season in your life with a diminished ability to recover from negative events. In my opinion you are inviting "tales of woe" to be part of your future if you borrow.
Others would disagree with me. They would point to the math and show how you would be much better off on borrowing instead of pulling out of investments provided a sufficient return on your nest egg. They may even have a case as you might have to pay taxes on money pulled out magnifying the difference in net income on borrowing versus pulling out in a lump sum. Here in the US, the money you pulled out would be taxed at the highest marginal rate. To help with a down payment of 50K, you might have to pull out 66,500 to pay the taxes and have enough for the down payment.
The third option is to not help with a down payment or to help them in a different way. Perhaps giving them a few hundred per month for two years to help with their mortgage payment. Maybe watch their kids some to reduce day care costs or help with home improvements so they can buy a lower price home. Those are all viable options. Perhaps the child is not ready to buy a home.
Having said all that it really depends on your situation. Say your sitting on 5 million in investments, your pensions is sufficient to have some disposable income, and they are asking for a relatively small amount. Then pull the money out and don't be concerned. You nest egg will quickly recover the money.