The only way you could do what you're asking is to "turn off" validation while creating the child records.
One alternative would be to have other child records display at the same time, while allowing the validation to either auto adjust them to 100% during creation or proportionalize them in some manner according to a predetermined algorithm; which might work best if the number of expected children has been specified in advance. I say the latter because you could specify the final percentages you want to have for each one if you know the number of them and do it in that order. If you don't, you'll get what I'll describe next.
The first child created would automatically be set to 100%. When you create the 2nd child and set it to say 40% during creation, the validation could automatically adjust the 1st child to 60%. You could set up a similar approach where when a 3rd child is created and a proportion specified for it, the remainder is adjusted according to the proportions already set for them. In the current example. If the 3rd one was created with 30%, that would leave 70% available to split between 1 and 2. 40% of 70, or 28% would be assigned to child 1 and 60% or 70, or 42% would be assigned to child 2.
This type of thing becomes inconvenient for a user when the numbers automatically adjust like this later. Every time you change one value, all the rest automatically adjust themselves proportionately; causing a user to need to make several adjustments to get the results they would like to have rather than just entering what they want.
So, the last alternative would be to have all the children to the side, including the new one that's being created, with a sum of the percentage totals below them at the bottom. Then, allow the user to adjust them as each one is being created, but require that they total to 100%.
I don't think there are any "optimal" solutions for what you want to do. If you allow the validation to be turned off, its unlikely it will ever be turned back on once all the children have been created.