depends... do you have a swimming pool? otherwise, i wouldn't. it would be terribly uncomfortable, i'd think....plus you have the possibility of very heavy rain or typhoons...
Yes, it's hot and humid - but you could choose a shady BBQ site in one of the country parks and start the party at 4pm. The BBQ sites in Tai Tam country park are generally shady (and they are a 5 minute walk downhill, if you start at Parkview, on a paved road).
Heat, humidity, mosquitoes i could still live with (or plan with or around). But in June it could rain heavily any time - so you'd need a very sound backup plan.
we had a bday party for my 4year old daughter in tai tam country park, last weekend, started at 11 and we stayed for several hours...it was not a sunny day though and we had to postpone once for the bad weather forecast....but after all, we had a great time