I gave up my plot when DD was born as it was too much with a baby, but now DD is 5 I have got another and starting all over again. If I'm going to be able to keep it going I know I need to make it fun for her, and be realistic about what I can achieve within the boredom threshold of a small child. I won't be able to spend hours digging or plant delicate little seedlings and expect them not to be trampled. DD need to be able to play while I get on with stuff.
I have put a swing on the plot and DD also has a little raised bed of her own to dig or plant in. She has her own little tools and gloves and I'm also going to build a little den for her. My plot neighbours also have a little boy of around the same age we encourage them to go and play hide and seek. So far she has been loving it and often asks to go there, but if the novelty wears off I might start taking flasks of hot chocolate and doing little barbeques....
Try and divide your plot into different areas and concentrate on one or two to get going and just keep the rest tidy to work on later in the year. On my site at least 75% must be cultivated, so on my plot 1/4 is grass with the swing and space for DH to build compost heaps (he hates 'gardening' but loves building/destroying/chopping/burning), I have covered 1/4 with black plastic to be dealt with later next year For a reliable crop this year I have dug another 1/4 and shoved in some spuds (charlottes). The ground is still very weedy and rough there but potatoes will still grow and they will also help suppress the weeds. On the remaining 1/4 I have planted currants, strawberries, raspberries and rhubarb, which are all pretty low maintenance and soft fruit is really expensive in the shops.
If you are going to buy plants then I would recommend strawberries, then cherry tomatoes once the weather is reliably warm. Get stuff that is a treat, you don't really want shitloads of cabbages (unless you really like cabbages). Otherwise get seeds they are much cheaper, carrots, spinach and salad are pretty easy.
Gosh thats a long post, sorry. I'm very excited to have another plot and cant wait to get going
Have fun