It's one thing to go on a nature walk and another to be able to identify what you see. Joining others who know is a good way to learn so look around for opportunities near you. Some useful organisations are listed on this page.
Maybe the first step is to consider what you most want to learn about? I started as a birdwatcher and it lead me into other things. Now my main birdwatching is done by listening to songs and calls as I go while I explore other subjects.
The main phone app I use while out is iNaturalist, it's free! But beware, it covers the whole world and you need to make sure it gets it right! Record your plant, insect or tree with a photo and check it when you get home, or, if you have plenty of data allowance you can do it live in the app, take a photo and check what it might be.
Some of the things I find are illustrated with my own photos on the pages. Most of the birds were photographed by Brian Slade or Brian Winter to whom I am very grateful.
Kids and nature
What do you need?
The best tool is a mobile phone with a good camera. You can take photos of what you see and identify them then. Alternatively, take photos and check when at home.
It's hard not to find birds, just listen and look, so you may want binoculars. Birdwatching binoculars need to be about 8x40, that is a magnification of 8 and an object lens of 40mm. The bigger the object lens the more light they let in but the more weight to carry. Mine are 8x42. As a rule, if the first number doesn't divide into the second number by more than 5 they may not be the best for birdwatching. Look here for more advice Which? Binocular basics. And here for advice from the RSPB. The temptation is always to get the highest magnification but don't fall for that.
To identify what you see the suggested apps opposite will help you a lot. If you don't have a mobile phone with apps you may need some books although at home, the internet can do so much nowadays.
Here are some ideas, most from practical experience.
Birds - Collin's Bird Guide is favoured by most birdwatchers - also an app with songs
Butterflies -Britain's Butterflies from Wildguides is a useful book with good reviews
Dragonflies - Britain's Dragonflies from Wildguides is another good value book I use myself
Wildflowers - I find this the hardest choice. There are so many flowers and identifying them can be a complex process. A good online guide to common flowers comes from Countrylife magazine. Otherwise, the Bloomsbury pocket guide should be helpful and easy to carry.
Insects - Like flowers, it is hard to know where to start. A Woodland Trust online guide is here and worth reading to understand how to start. If you want a book the Collins Complete Guide to British Insects is a good choice.
Fungi - You could start here with this UK guide from the Woodland Trust. If you want a book Collins Fungi Guide is comprehensive. I warn against trying to find edible mushrooms but there are books that will help you do that - at your own risk!.