Takamatsu, birthday for kid, idea?

Hi, thanks for helping me.
We will be in Takamatsu on my youngest son's birthday this year, turning 7. early July.
I was wondering if there's any kid friendly fun things to celebrate there for him?
He loves any type of ball.. cake..
We have a whole day free.. I know there's the garden but that's not something he will Love..
do they have feeding the fish inside the garden?
What about a great restaurant with cake to celebrate? cafe? where we can get a really fun birthday cake?

Thanks for any tip..
if not, my fall back plan is to eat dessert from shop to shop.. :)

Hard to answer without knowing your son and what he likes, but there are a few things that every kid likes in town.

I don't see why he wouldn't like the garden (I assume you mean Ritsurin Garden, right?)
Every kid love gardens. And while he can't run around everywhere, there's still enough places where he can, and if that's his first time in Japan (is it?) it'll be new and unique enough.
Really, 7 years old are interested in anything as long as you introduce the thing to them, enjoy it with them.
And yes, you can feed the fish and the turtles there (with food that is sold on site, you can't give them anything, obviously).

Other spots for kids:
- Sanuki kodomo no Kuni. I'm not sure if you can go there with public transportation (it's right next the airport though), but it's an amazing facility for kids, with both indoors and outdoors things for hours of fun.

- Megijima is famous for a cave, that's super tacky because it's full of plastic Oni (it's supposedly Momotaro's Oni cave). I would never advise an adult to visit the cave (although it has some nice art from Art Setouchi), but it can be fun for kids, like a small adventure.

- Ogijima is a great island too for both kids and adults. The village is some sort of maze, and there is a nice walk to the lighthouse.

(note that if you go on week-ends, art from the Setouchi Triennale is open, and some can be fun for kids - on the other hand, most of the art on the more famous islands of the area is not very kid-friendly)

- In Sunport's Symbol Tower, there's also a place called E-Topia with small "science experiments", and things with video and such that can be fun for kids (it's a small place though, it'll last less than an hour).

For a birthday cake, you'll have to go to a pastry shop. There are some all over town, some where you can eat inside, some not. The shotengai (the covered shopping streets downtown) has a few.
(You won't find birthday cakes in restaurants though)

I hope this helps, if you have any other question, do not hesitate.

Thank you for your help.
I was more thinking about a restaurant that maybe sing a song to celebrate birthday, like the ones they do here in US?
I am sure cultural differences maybe that it is not usually celebrated out in public.. I just don't know.. :)

Takamatsu will be later during our trip and I think they might be "cultured out."
So, a garden would normally be great... but not after garden and garden from city to city. :)
I saw the guides about visiting the other islands surrounding the area. But, many in our party are prone to sea sickness.. so, I am weary of that route. Do you know if the ride is mostly smooth?

I will try to get a cake from a pastry store instead of in a restaurant, like you suggest.
Thank you again.

Depending on how many people you are travelling with, in a garden you could organize some sort of game for him. Give him a map and then let him find the points in a map and in each point is person with a task that he has to accomplish. The last person can have a cake and a little present.

Personally I am not at all for strangers singing happy birthdays in a restaurant.

The inland sea is smooth