Do you want to serve your guests more food than just cake? If you'd rather live off pizza crusts and protein shakes than deal with potential picky eaters, it's best to avoid commonly considered mealtimes. Ending a celebration by 11:30 am or beginning after 1:30 pm gets you off the hook for lunch, and if you let your guests know they'll be leaving before 5 pm, they probably won't expect any dinner bells.
Is your child a napper? If the answer is yes, or if they're phasing out of naps but most of their peers still take them, you want to pick a time when your little one will be awake and refreshed, and end it before exhaustion sets in and the meltdown potential is rapidly rising. (Both for you, and the children *laugh*)
For toddlers who might not be used to spending extended periods of time with other children, limiting the celebration to an hour is a great idea. Pre-schoolers, who are used to playing around others for several hours, can last a bit longer. (I'd suggest 1.5 hours - some free play, a Group Activity, and then of course - cake!) Holding parties for these ages between 10 and 11:30 am is my favorite time.
As children get a bit older, they can handle longer celebrations. For friends over 4, up to about 10 years old, 2 - 3 hours is wonderful. The most popular time for parties for that age group, in my experience, is between 1:30 and 4:30 pm. I would definitely suggest providing some sort of snack at the 3 pm mark!
After 10 years old, children are a bit more self-sufficient (and want to feel very grown-up!) so they may want an evening party or a sleepover. (If the latter is the case, just be sure to be clear about morning pick-ups. I advise a window to accommodate both early birds and night owls *wink*.)
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