Hi OP
I think Malta is a bit like marmite; some people love it, others don't.
I'm in the first category, I love the place & can't wait to go back. We always stay in St Julians, close to Spinola Bay. There are dozens of lovely places to eat around the bay and it looks so pretty at night. I'd suggest avoiding the centre of Paceville. Although it has a bowling alley & cinema, it's also full of clubs & bars for over 18's.
The pavements are pretty narrow all over the island. I don't think it's the best choice for parents with very young children, unless the plan is to stay on the sandy beach at Melleiha. Popeye's village (they built the set for the Robin Williams film in 1981 & left it there) is also in Melleiha. The public ferry terminal is also at the north end of Malta....it's much cheaper to get the ferry to Gozo from there. I'd recommend visiting Gozo for a day. Go on a jeep safari or quad bike day with a guide to show you the best spots.
Valletta is a wonderful city to visit. I'd recommend a private boat tour of the Grand Harbour. I always check that a cruise ship (or two!) isn't due on the day(s) we visit as it can get exceptionally crowded, even with just one cruise ship.
Churches are not everyone's cup of tea. Malta has over 300! If you only visited one, make it St John's Co-Cathedral in Valletta. The Caravaggio is in the museum there.
Mdina (the silent city) is breathtaking. The kids would probably enjoy the Malta 5D experience, which tells the story of Malta back to the days of the Knights of St John. Rabat is very close by, definitely worth visiting St Paul's catacombs. IMVHO, Crystal Palace (Is-Serkin) in Rabat make the best pastizzi.
Every village & town in Malta has at least one patron saint. There is a 'festa' in the village/town to mark the feast day. The towns are decorated with huge banners & the church will be lit up. There are marching bands and LOTS of fireworks! They are great fun to watch but can be a bit rowdy. The festas run mostly between May-September.
Depending on the age of your children, you might like to spend a few hours at the Ta’ Qali craft village. You can watch glass being blown as well as see other craftspeople at work (lace, ceramics, jewellery etc.). I love walking round the Mdina Glass shop, but I wouldn't take a young child in there if you paid me!
There is a water park on the island which is also a bit marmite. To be fair some of the older chutes are showing their age. It had an injection of investment in 2019 and some brand new chutes/attractions were added. The place is a tad worn, but perfectly pleasant for a visit. I'm not a chute fan myself, I'm more of a lazy river kind of person. My son recommends wearing a rash vest or t-shirt.
The National Aquarium is good, but would only take a couple of hours at the very most.
I can see the taxi app has also been mentioned (I agree, it's really good). The public bus service on Malta is also super cheap and very frequent, since it's the only kind of public transport on the island (not the hop on/hop off buses). I'd recommend either getting a taxi or bus over hiring a car, the roads can be a bit crazy!
I just love the hot, sunny weather in Malta during the summer. The prickly pears growing at the side of the road always make me smile. They look like how a child would draw a cactus plant, it's hard to believe they grow like that!
I hope you have a wonderful holiday. 