Okey dokey. The first (but certainly not the last) of MN's gardening obsessives checking in.
Amongst the weeds, have you got any salvageable plants - shrubs or perennials (which wowuld be coming though about now)?
First, obviously, you need to get rid of the weeds. If you have the time, you can just pull/dig them out. There's also the chemical warfare approach if you're in a hurry but, frankly, by the time you've sprayed and waited for them to die, you could have just pulled/dug them out.
For OP' sunny front garden, I would suggests lavender and thyme and other shrubby herbs like rosemary and santolina. They will provide lots of sensory experience for the children. There are lots of other low maintenance perennials - such as aquilegias, geraniums (not pelargoniums), astrantia and alchemilla mollis which will just get on with the job. Bamboo is low maintenance and will make a good screen against traffic noise but be careful to get a non-invasive one (such as black bamboo, phyllostachys nigra) as others will quickly run amok and choke other plants.
Is the back garden large? If it is, you could create a veg patch where the children could grow things - radishes, beans and peas are easy. So are flowers like nasturtiums, clarkia, godetia, cornflower and love-in-a-mist.
A living willow wigwam or maze would be fun, if you have the space.
Go to boot fairs, scghool fetes ansd WI markets to get cheap home-grown plants.