Do you mean for you as the beneficiary or for you to pay the premiums for their benefit should one of them die? If you're just paying the premiums for them to take out their own policy, there shouldn't be an issue (probably not enough to get anywhere near gift tax) - probably easiest for them to take out the policy and you give them the money. If you want to be the beneficiary, the life company will want to know why you need the policy - you will need to show you have an insurable interest in their lives (e.g. if they looked after your children for free).
Also, are you sure they need/want life insurance? Is it to cover loans they have outstanding? It will be pretty expensive to get long-term cover for them so you need to be sure it's necessary in the first place and don't buy too much.