I would leave out "if you need my help" and make it sound like you assume he will not try to have a birthday party without you. To me, it sounds unsure of your presence at your own daughter's birthday. If he makes a move to exclude you, then the whole tone of things might change. As for monday, I am assuming that is your day. My take would be to nail down the party and making sure you are involved first. If he includes you there, then I would go ahead and include him in the outing to Tahoe. Ooh, I love Tahoe!
