There are a few ways to tackle this depending on how you would like to address it.
1. Being subtle and addressing the envelope to the adult recipients only.
This method isn't foolproof though, as some guests may not get the hint that children are not invited.
2. Being blunt and straightforward.
This will definitely get the message across but you may have to be cautious when dealing with sensitive guests.
"Sorry, no kids."
"Please note that this is an adults only wedding and reception."
3. Asking in a polite manner to respect your wishes.
"We respectfully request no children under 16 at the reception."
"As we would like to keep this wedding adults only, we kindly request no children be brought to the reception."
4. You can write it in a poem.
But realistically, being honest is the best way to go about it.