I held a party for my son on 17Dec. The combination of adult and children is similar to you. I think it's ok. As other BK mom said, just for fun and have a gathering together.
1)50 people(many children) about $1500(not includes cake)
2)$120 basic charge for decoration , cap,fork,etc.
3)it provides free 10 presents,but $12 for each 禮物 for extra.
4)一歲is OK la,more important is she is happy to have party.of course 3-4 years old will be better to have more friends such as classmates ,moreover, they can enjoy and play the games.
I bought cake from Maxim's 'coz I have discount and I like their cakes more.
I think 聖安娜 cake is ok. I know some friends ordered birthday cake at there.
I didn't pay much for the party as not everyone ate. There were 28 adults and 7 children and I paid HKD800-900 (just for the food and drinks in McDonald's). I know my friend's birthday for 60 people was around HKD1800. So you can assume maxium $30 per head if everyone eat.