Looks like you've got a really good start there
I love you little construction areas for Leon! I think just tidying it all up and setting it up nicely ready for the summer will make a big difference (dont' forget pics when you are done!).
I think painting all or part of the fence would be nice - could help give different zones to the garden. The shed coudl look really cool if you went a bit wild and had fun with some paint, hung some brightly coloured planing boxes with flowers etc - if you did that first you may find you didn't need to do anything to the fence, or you could put up temporary decorations that could be changed out. Lots of fun ideas on line!
I'd maybe do corner of the fence as the kids are with a chalkboard and maybe one of these cool DIY water play activity boards:
I love this kind of look:
Re the grass, yes go grab a bag of grass revitaslier (not sure what it is called in English - I've direct translated that from French
) quickly as you need to do it now and probably some fertilser too to perk it up. I'm cross as I put my seed down this week, just when it stopped raining at least every 2 days so I'm not sure it is going to work
You coudl also look at putting in some pavers (there are some awesome DIY ones that you could have fun making with the kids) as paths to the most used areas.
That is cool if the rabbits don't poop everywhere too!