I am making the assumption that an encounter made up of two medium half-jellies (or four small quarter-jellies) should not be much harder than an encounter made up of one large standard jelly. In other word, a jelly is not tougher if it's already split.
This is because the jelly is expected to split as the combat goes on, and as a result the increased difficulty of fighting multiple opponents is already accounted for in the jelly's CR (which is quite a bit more than what we get by just running the numbers according to the DMG's "Creating a Monster" chapter.) It might be a bit more, because the jelly normally remains unsplit for a bit of the fight, but not much.
So since the XP value of a regular jelly is 450, the adjusted total XP of two half-jellies should be 450 as well. The encounter multiplier for two monsters is 1.5, so the actual total XP of the half-jellies is 300. So the XP value of a half-jelly is 150.
By the same reasoning, the XP value of a quarter-jelly is 50.
In terms of CR, rounding up to account for the possible increase in difficulty discussed above, that makes the half-jelly a (weak) CR 1 and the quarter-jelly a (possibly strong) CR 1/4.
Since the quarter-jelly cannot split any further, its CR is not affected by anything other than its immunities and resistances. So we can double-check using the DMG's math.
The quarter-jelly has 11 HP, possibly an effective 22 because of the immunities and resistances, though it does not matter, its defensive CR is 1/8 anyway. Its 8 AC is 5 points lower than expected, so it drops two steps to somewhere below CR 0.
The quarter-jelly does 12 damage on average with its attack, so its defensive CR is 1. Its attack bonus is as expected, so nothing changes.
This gives us an overall CR of somewhat less than 1/2, which confirms the above result of "strong CR 1/4".