Planning a children's party at the park is a GREAT idea - the only thing is, like you mentioned, the weather can be funny - especially in April! so...I'd say, plan for the park, but have a back-up plan - if that has to be your house, then so be it! But in regards to what you need to plan for...plan for the children! Don't set your guide to how you "do" this party around everyone (the adults are accessories - they are not the real "invited guests") - and remember you don't have to go overboard. You can go to a nice Dollar Tree & find all that you'd ever need for a children's b-day party - they have all your design & paper needs, balloons, "loot" bags (if you plan to do these), candy & snacks, get big bottles of juice & water to pour in cups rather than buying expensive juice boxes, etc. Make yourself a list of what you'd like to have/do at the party. Don't over do it for children's activities - especially if you'll be at the park, cuz they'll be playing there. But if it ends up being at your house, plan for coloring activities, musical chairs, maybe an easy do-it-yourself craft that they can take home with them. Again, balloons are ALWAYS a hit - can't have enough of them...they can make a game from who can pop the most...who can keep theirs from hitting the ground, etc. You can also find a HOST of ideas online and again, you don't have to spend a lot to make this a memorable occasion. Oh and on that note, your daughter probably won't remember any of it! So...just make sure to take lots of pics so that you can show her later on in life...she'll love to see how it was when she was a "baby". Good luck!