With any party- pray for peace, prepare for chaos! (Especially if your guests are 4 and under.) If parents are staying rally them into helping (eg breaking up squabbles or taking children to the toilet). If you don't have many adults there make sure you know all the children's names so they know who you are talking to at any time.
Be aware of the exits - make sure no kids wander out.
Make sure you have a LIGHTER, CAKE KNIFE and GARBAGE BAGS :)
Don't open gifts at the party, take them all home to open.
Make sure you have plenty of water or access to drinking water for thirsty kids.
Sunscreen if outside!
Delegate a partner or friend to take lots of pictures in case you forget in the moment.
It doesn't have to be perfect. Don't worry about the details of the party, enjoy the celebration of the birthday child.
Delegate jobs to your 'party squad' (partner, friends, the birthday child themself) to help you get it all organised for the day, don't try to do everything yourself. This can include helping with things like filling the party bags or preparing the buffet.
Eco friendly party bags could include: seeds to grow in the garden, simple origami set (colourful paper and instructions), organic chocolate or candies. Or go for a zero plastic party bag.
Biggest tip? HAVE FUN. Enjoy celebrating your little ones birthday, connecting with their friends and other parents. It will pass in a blur, be as present as possible and enjoy :)