don't make it a question, just send them the adjustment
"we look forward to having you, I noticed that your guest count is wrong, here is your updated reservation. thanks!"
It's kind of ABB's fault, they totally hide the rules from guests. It needs to be in your listing text and in your rules and then you need to, in a very not bothered way, send them the correction. Immediately, so that they can pay correctly or cancel.
And no, I wouldn't keep going out and buying pack n plays. Many people bring their own, just tell them what you have. A basket of toys, a box of cheerios, some outlet plugs are all baby extras without you having to stock an entire fleet of baby gear.
My standard answer on early checkin these days (if it is available) is yes, we're happy to commit to an early checkin for you. The fee is xxx (usually half of our nightly rate) because we'll be leaving the space unoccupied the evening before to make it available for you. If it has value to them then they can pay, if it isn't worth the money then they can find something else to do until checkin time.