On the first sentence: The IRS does have a limit of $15,000 in gifts given per person to a recipient (no limit to the number of recipients) before a gift tax is triggered. So the purchase limit of bonds is one question the OP has, but it seems there is a question of the limit amount of gifts as well.
To answer that question: you and your husband can each gift up to $15,000 to each of your children each year without triggering a gift tax. If you and your husband each gift $10,000 to each child (for a total of $20,000 per child) you aren’t going to trigger the gift tax since you’re each gifting less than $15,000.