I think you need to physically see and hold things to fix them in your mind, rather than the words/numbers.
If you aren't using it already, squared paper is a lot easier than lined, especially the large 1cm squared paper as is used most in primary schools.
Cutting up shapes to represent pizza/cake works for fractions.
If you had 4 children, they could have a quarter of a pizza each, which could be written as 1/4 - they have one piece out of the four pieces you've cut.
To make it easier for them to eat, you could cut each slice in half - they'd have the same amount of pizza each, but instead of one slice, they'd have two smaller ones.
You've then gone from 1/4 to 2/8 each. They have the same quantity of cheese and tomato pizza.
If one child is much smaller, she might only be able to eat one of the smaller pieces. That would mean three children had 2 pieces - 3 x 2/8 (or 3 x 1/4) = 3/4 of the pizza and the little one had 1/8 of the pizza and her other 1/8 went into the fridge. The amount of pizza that has been eaten is either 3/4 and 1/8 - or, 6/8 for the older ones + 1/8 for the little one; 7/8 of the pizza.
If you only cut the little child's pizza slice in half and left the other children's as quarters, they could be arguing about who has had the most pizza if she then decides to have her second little slice. 'It's not fair, she's had two pieces and I only got one!'.
To prevent pointlessly annoying arguments like that, it's easier to cut everybody's pizza to the same size as soon as it's out the oven - this is why it's sensible to find the easiest common size/fraction for comparisons.
If you had two older children and two little ones, you'd do the same, 2 children having a quarter (or 2/8) each, 2 children having 1/8 each, 2 slices left in the fridge.
The older children have had 2 out of 8 slices (2/8), a quarter (1/4) of the pizza each, or between them, 4 out of 8 (4/8) slices, which is the same as 2/4 (two quarters) or half 1/2 the entire pizza.
It's far easier to see it and do it with either pizza or by cutting up a circle of paper to show the pizza and put the pieces together (before they're eaten!). You will also be able to do exactly the same if it were a square pizza or a rectangular pizza.
It's a very longwinded way of explaining it - but doing it physically so you can see and feel the actions makes sense. And once you can see what is meant, writing it in numbers and equations makes sense - this gives you the skills to be able to work with 3 children, 5 children, 10 children and a giant pizza or two medium ones.
It also gives you the skills for working with algebra in time - because, in algebra, you are using a letter to represent 'something' - this time it's pizza, but it could be cake, pies, ice cream, a bag of sweets.
Algebra could be visualised as fruit. Apples and bananas. 2a +3b = 5f: 2 apples and 3 bananas is 5 pieces of fruit in the fruitbowl.
Checking your answer 'there are five pieces of fruit' could be done by starting with the full fruitbowl instead. 5f - 2a = 3b: 5 pieces of fruit, taking the 2 apples out leaves the 3 bananas. Or you could take the bananas out, which could be written as 5f - 3b = 2a.
It won't take long until you can visualise what is being asked = when that happens, all of a sudden, Maths isn't so scary anymore, it's just a lovely, simple way of explaining things that take ages to type.